Here is a tutorial that I'm working on, to help people to create custom subhood templates:
http://www.modthesims.info/showthread.php?t=420196

Unfortunately, some people are having problems posting in that forum, so this is the place to ask your questions or post comments. You'll have to join the Moo Tools social group if you want to post here, but it should be accessible to everyone.

It is providential that you posted this today. I've spent the last several days reading old tutorials and threads in preparation to build a Mega-hood to my personal satisfaction. Your promised tools and tutorials are already making that project look a lot easier. As a thank you, I'm willing to act as a guinea pig. Not a very good pig, I've already done dreadful things with your existing tools, but I'll try hard.

Question: Will this allow us to add a suburb to an existing Mega-hood?

Good question. I haven't tested adding subhoods to existing mega-hoods.

All of the existing mega-hoods were created in an odd way, by replacing existing suburbs with Maxis-made neighborhoods which had their IDNO records replaced, rather than by creating proper subhood templates. I seem to remember some warnings about adding new subhoods to these mega-hoods, but I don't know why those warnings exist. If there's something odd about the mega-hoods, this new procedure for making subhood templates may not solve those problems.

The best way might be to create new suburb templates from cleaned versions of the Maxis-made neighborhoods and then create your mega-hood from scratch. I haven't seen any limit on the number of subhoods when using this technique. Plus, you should be able to have multiple mega-hoods with different sets of combined neighborhoods, as long as they don't use the same base neighborhood.

Here's a link to Tarlia's cleaned neighborhoods, which I've been using in my testing.
http://meetme2theriver.livejournal.com/36433.html
If you want to know what she's done to the shipped neighborhoods, it's all documented in her blog:
http://meetme2theriver.livejournal.com/29134.html

If you decide to try to create new suburb templates and add the suburbs to an existing mega-hood, be sure to backup everything first, in case things don't work as expected. Let me know if you run into any problems.

I don't want to add to an existing 'hood, I was asking because some people might want to. I want to create a new merged base game hood based on OFB suburbs with custom suburbs added at creation time. I could do that the old-fashioned way, but your method seems better at first glance. I'm awaiting the tutorial on creating occupied suburbs and testing the method already presented. No pressure, my play in an old Mega-hood has reached an interesting turning point and I have the latest Sims3 EP installed, but unplayed.

Are you asking for a tutorial on how to create an occupied suburb from scratch? If so, that's explained in the custom subhood tutorial.

Or, are you asking for a tutorial on how to change an existing occupied neighborhood into a suburb which can be added to any neighborhood? I have a brief synopsis of the technique in post #15 of the custom subhood tutorial, but I am planning to write up a more detailed tutorial soon.

[Update:] Tutorial now available:
http://www.modthesims.info/showthread.php?t=502306

Thank you for the tutorial, I have been looking for some way to do this for awhile. The tutorial is easy to follow and the BV subhood I created works just as it should. I am currently building a university and will test the creation of custom townies and NPCs (I have empty templates installed). I do have a quick question, though: I like the Maxis Downtown template, but I always find myself tweaking it a bit. Would there be any (safe) way to edit the template permanently? I am thinking that I could just add the neighborhood from the Sims 2 Nightlife/ . . . /Neighborhood Template to a clean AnyGame, make my changes, and then reinstall it in the Neighborhood Template folder. What are your thoughts on this? Will it irrevocably damage my game (although I will back up first)?

First, let me thank you for letting me know how things worked for you. I'm glad that you found the tutorial easy to follow.

Yes, I believe that the process that you want is almost identical to the one in the tutorial, and that you should not have any problems with conflicts as long as you follow the steps to rename the folder and to change the (parent) Name.

1) Make a copy of your D001 folder from Program Files into your Neighborhoods folder for your empty AnyGame, under My Documents.

2) Rename the copy to a unique name. Rename all of the files inside the copy to have the same unique name as the file prefix. Because there are a lot of files, you'll probably want some sort of rename utility.

3) Use SimPE to change the (parent) Name of the neighborhood. You won't need to change the Neighborhood Type, since it should already be a Downtown.

4) Make your changes to your new Downtown area in the empty AnyGame. Be sure to give the new Downtown area a new name and description in the game, so that you can tell them apart.

5) Once you are done, you should be able to move / copy the new folder into the NeighborhoodTemplate folder right beside the original D001. As long as you've done steps 2 and 3 correctly, there should be no conflict between the two. Note: I would not add the original Downtown to the same neighborhood as your modified Downtown.

Please let me know how it goes.

Thanks Mootilda for getting back to me so fast! I will keep you posted.

Just a note as to why I haven't replied sooner to your reply. My old computer died, so now I'm starting over from scratch. And having headaches from hacked objects, that never gave me problems before, and even base game animations throwing errors. It will be a while until I'm ready to build a neighborhood.