I've never used anything from Cawster, so download responsibly if you decide to go that way. If you'd rather start from scratch (just an editable world map), you can find most or all of the official worlds at but I wouldn't venture a guess on how to get the townies imported. #2 is the continuation thread and has a pretty good summary of the first thread at the beginning: There is a rather lengthy discussion of this process (looks like it took place over about 8 years) on these two threads. What you're proposing sounds like you'd need to edit the world file, which needs Create-a-World (and probably a couple of other tools).
I'm going to assume that you're talking about the terrain outside the lot boundaries inside the lot boundaries can be edited in Build Mode.
