Adding Roads to an Existing Neighborhood:

Update: The following program replaces the need for this tutorial, and will keep your decorations intact:
http://www.modthesims2.com/showthread.php?t=279992

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Tutorial (now obsolete):

I managed to add roads to the existing Strangetown neighborhood, without destroying the existing families and their relationships!

Here's how... just make sure to back up your Sims2 directory, in case you have problems:

Make the changes to the Sims2 SC4Terrain Strangetown.SC4 file using SimCity 4 Rush Hour (or Deluxe, I suppose). Then use the changed terrain to generate a new temporary neighborhood.

Using SimPE, export two things from the new neighborhood: Neighborhood Terrain Geometry and Neighborhood Terrain. Then import those files into the real Strangetown, with reload, replacing the existing terrains.

Unfortunately, this will delete the decorations in the neighborhood, but they were very easy to replace... well, all except that one rock in the middle of the road circle near the Specter residence. Don't forget to add smoke to the crashed spaceship.

In addition, I had a couple of pieces of road disappear near the Curious place and near one of the empty houses. However, this was very easy to fix by just going into those houses and making a change to them, such as moving a door temporarily, then saving. The road pieces reappeared.

Also, the landscape is a bit lighter than the original one. I have no idea why this is or how to change it, but it doesn't seem like a big deal.

Finally, don't forget to delete that temporary neighborhood when you're done.

Voila!
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A variation of this technique can be used to fix existing roads in a neighborhood, if you still have the original terrain available, or if you have a backup of the neighborhood package from before the roads were mucked up.

I used this technique on a neighborhood where the roads and landscape had become "stepped" because of changes made when adding and removing lots.
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I also used this technique to "port" the Life Stories neighborhoods to The Sims 2. For some reason, the terrains that EA includes with Life Stories don't actually match any of the neighborhood terrains. Since I wanted exact copies of the neighborhoods, I used this method to replace the terrain of a newly created neighborhood with the Life Stories terrain.