I'm still a noob in TS3 CC creation and I have no idea in how turn a contact eye into a default eye for my game.
The eyes I'm pretending to modify are these:

It's a conversion 2t3 made by WunderSims.
I'm not thinking to post them, just use in my gameplays (but maybe I do )
*Sorry for bad english*

I always make my default eyes by this method. Simply said its a matter of putting fitted image from one picture to the next. There is no mesh converting or anything of the sort, just pictures being moved, edited and renamed correctly.
I would go about it by finding some other default eyes first as base, that because you need the correct instance names. In my example I have some default from brntwaffles. You open the package with s3pe. In the picture below you see the red square being the instances, which are personal names of everything.

To edit your picture you need to export it, that is marked by purple square. For image editing you can use Gimp, which is free.
When you have edited the picture, you need to import it back. I usually use replace which is marked in blue, but you can also import it back through as is show on picture below in green. But do keep in mind that if you use the "green" method the names must stay the same for import to be successful.

My exports from Gimp are usually .dds bc3 with mipmaps, but I'm by no means an expert in those.

Extra: for gimp I believe you need a dds plugin, I don't know if current version has it included or if you need to download it yourself.

Thank you so much, but what are mipmaps? :D

P.S.: I'm so happy that someone answered me, I thought no one would. I want to thank you so much, I just can't—

Hey its okaay I'm glad to help out!

I don't have a good understanding of what mipmaps are, I just learned to export them so I do. But wikipedia sais this:
Mipmaps are used for:

Level of detail (LOD)[6][7]
Improving image quality. Rendering from large textures where only small, discontiguous subsets of texels are used can easily produce moiré patterns;
Speeding up rendering times, either by reducing the number of texels sampled to render each pixel, or increasing the memory locality of the samples taken;
Reducing stress on the GPU or CPU.

It's basically smaller and smaller version of your original texture. So I just do them, since I also see like all other creators make them too. You can see an example here below. That is some hair I'm re texturing. The original texture has those mipmaps, marked in green. (I'm using gimp in the picture)
When you export your texture in dds format you choose those other options I selected in green.

Make sure to click the make new mipmaps options, otherwise the textures wont be fully updated to changes you made.
Also I select bc3 as I found lower options eat up the quality of the image.

You don't need mipmaps while editing, its just something you do when you are done and are going to export the image. Then when you export the image the mipmaps are generated and if you freshly open the image again in Gimp those new mipmaps are there. All the edits are done on the main picture and not mipmaps.

@Akasimsgen wondering if you managed?

Thanks after 3 years! It's hard to find any tutorials but your answer helps!