Hey, Sims 3 noob here!
I’m trying to figure out how to completely swap a general Sim’s model with a generic object like a tree, a crate, or even a custom made object. Basically, I just want the Sim’s equivalent of the once popular Source Mod, Prop Hunt: no bone tweaking, just non-animated objects moving around with perhaps some direction and rotation of where they’re aiming.

I’ve made quite a few attempts by trying to adjust an already made .package meant to model swap another character; however, if I were to import and replace my custom mesh into said character, a ton of snags happen and I can only reach the Master Control CAS clothing section before the game ultimately crashes and shuts itself off.

1. I needed the same number of Groups for a full body replacement regardless of how simple the replacing object mesh is. A workaround I found was to make a dummy mesh within the working mesh just so it imports without any error.

2. I know for certain the new mesh might need a bone or two. Unfortunately, by attempting to transfer bones from the original object into this Frankenstein mesh, it makes the whole model disappear and only a few bones partially made it.

3. I tried to just add the mesh as a group of its own while shrinking the original meshes in MilkShape; however, it wouldn’t allow three groups to import and deleting the lesser important group takes me back to Sims 3 crashing again.

4. I’ve tried adding the same bones from the original mesh to the new one in Milkshape, but how that program selects vertices almost made me want to quit entirely!

I normally see simple model swapping to be a cinch with most games; so easy, it can be done with a few Action Replay codes in some cases! I now understand when it comes to Sims specifically, that’s a whole other can of worms to open. So how can one turn a family of Sims into a family of sentient cardboard boxes?

I do have S3PE, S3OC, and the Blender GEOM plugin, but I’m more of a loss with those three over TSRW and it feels like I’m already overcomplicating what may ultimately be a simple problem.

The general concept of an object as an outfit is definitely possible, but the main setback is actually hiding the head. While TS4 has some flags for hiding face parts for costume masks and such, I don't think TS3 has an equivalent.
The sim body isn't one model, but many separate pieces- a hair, face parts, top, bottom, and feet. Clothing and shoes just being replacements for the last three that include the visible body pieces. You could get 90% there using the bald hair, a full body outfit that is an object, and invisible shoes- then you'd have a floating head that can't be swapped in the same way.

I'd gladly describe how I'd make a box into an outfit, for example- but you may just want to check if there is a way to conceal the head that works for what you have in mind first. Simbots and mummies have no head, as well as the grim reaper, but have their unique traits and such to deal with. Or, if you don't mind affecting all sims, a default replacement of the head is an option. You could also clone one of the hats that covers the entire head (the costume masks) and give it transparent textures to see if it removes the head- but I have a feeling they're just big enough to conceal the head without making it invisible

Maybe I want to start with something stupidly simple just to test out the execution: turning an infant, which has only two Groups as its overall model, into a cube freshly made from Blender. To put it as straightforward as possible, I want the Sims to interact with a plain grey cube exactly how they would interact with an infant. That’s probably the closest thing to a first step I can come up with. And no, I don’t mind altering the default assets for this mod: in fact, that’s something I would prefer.

Thank you for responding!

Sounds like a plan! Just wanted to make sure before launching into the somewhat time-consuming process.

CAS Parts are a lot of fun to work on now thanks to the Blender GEOM plugin, Thornowl's tutorial, and the comments there by Lyralei. So if you ever want to learn more than I cover here, I highly recommend checking out that thread (which also covers some TSRW specific parts, if you'd prefer). I'll try to be brief just in case you're already familiar with some of the concepts, but feel free to ask for me to elaborate on any steps

If I were to make a cube-baby, it would go something like this:

• Use S3OC to clone the buBody CAS Part. You can untick Renumber for it to be a default replacement, or leave it ticked for it to be an additional outfit.
• In S3PE, select buBody_lod1 and buBody_lod1_1, right click, Export > to file.
• In Blender (I use version 2.93, but any that GEOM tools supports should be similar), import both GEOM. You may want to rename the meshes to LOD1 and LOD1_1 to keep track of the groups.
• Using the baby mesh as reference, resize and reposition the cube as desired.
• To deal with the second group, Add > Mesh > Plane and scale it to be very small and hide it inside the cube. On an outfit that supports transparency you can make it invisible, but the baby geom uses an opaque shader so it's simpler to just conceal it.
• Triangulate Faces on both objects- reset the normals if they get weird shadows.
• Finding an appropriate bone to assign is the tricky part, as most will attempt to deform the mesh to perform animations. Having a look through each Vertex Group in Weight Paint mode, many of the bones used in this model are unnamed- but 0xcd68f001 is promising, because it's weighted at 0. Hopefully that implies it isn't used for anything. 0x6c03ce1b, the midpoint of the swaddle, is a good second option.
• Add the vertex group to the cube and paste the name. Select all verts and click Assign. Repeat on the 2D plane.
• Object Mode, select LOD1 and shift-click the cube object in the Scene Collection. In Scene Properties > Sims 3 GEOM Tools, click Transfer GEOM Data
• Repeat for LOD1_1 and the plane.
• Make sure all transforms are applied to both meshes, as GEOM export will not preserve them. CTRL A in Object Mode > All Transforms.
• Select the cube, Recalculate IDs, then Export GEOM- name it LOD1.
• Repeat for the plane, name it LOD1_1
• Weird step- there's currently a small bug with the GEOM plugin that can be fixed like this- grab the SimGeomEditor from here, open either buBody geom with it, File > Export MTNF. Then open your new LOD1, File > Import MTNF that same file. Save and repeat for LOD1_1.
• Back in S3PE, open the package from step 1 and right click Replace... lod1 and lod1_1 - without the fix above, the data preview in the left pane would be corrupted. With meshes this low poly, you can use these same files to replace 2 / 2_1 and 3 / 3_1 as well.
• I made a plain grey 512x512 texture with a white alpha channel, saved in DXT5 with mipmaps, to replace the multipliers (_m) and one with a grey alpha channel just to use as a dummy normal (_n). I'll also use a little black square for the specular (_s) just for testing purposes. Right click Replace each of the textures.
• One last thing if it's not a default- the part is named as if it's unisex but it's actually not, there's a separate bfBody. I selected the CASP in S3PE, clicked Grid on the bottom tab, and added the Female flag to AgeGender.
  I left the blue preset as is.

It seems pretty long-winded at first glance, but once you're familiar with the steps I can promise it's not very complicated
I went through the steps as I wrote it out in a single sitting. Had one hiccup with forgetting to assign the bone to the plane, but here's the hilarious little cube baby at the end:


Nature is beautiful.

Note, there are some weird shadows in CAS that aren't present in live play, which is something that can be polished out with a proper retexture and changing the shader to your preference and such.
Some other useful tidbits-
- The empty morphs on the baby outfit will be very useful for repeating this on the older sims. Export any of them and you can use it to replace the morphs in whatever outfit you use as a base in the future.
- GEOM can use pretty much any shader, so you can replace SimSkin with a standard object or accessory shader if you wish.

Hopefully I didn't miss anything, and feel free to experiment and mix it up with other methods!

Thank you so much for this little guide! Unfortunately, I gonna have to put off playing Sims 3 for a while. My computer is losing space, so before I get back to investing my time in Sims 3, I have to save up to buy an external hard drive. That way, I can install and make more nonsense to my hearts content!

A beautiful thing about help forums like this, I can search for this thread at anytime to find your solution whenever I’m open to messing with Sims 3 again! So again, thank you!