It is very possible! Doing that saved me a lot of heartache when I was making the beginning of my story. Here is what you do:
Find the folder where you actually have the game installed. This is different from the folder where you would put custom content and the like. If you did a basic install, the path on your computer would most likely look like this:
C:\Program Files\EA GAMES\The Sims 2\TSData\Res\UserData\NeighborhoodsFolder N001 = Original Pleasantview, N002 = Original Strangetown and N003 = Original Veronaville.
Copy the neighborhood you want and paste somewhere else, but DO NOT delete or cut it. I would paste it onto your desktop for ease of finding it later.
Rename the folder something like N007 or a number that is not the same as any of your other neighborhoods. All of the files inside that folder will also need to be replaced. If you have some time, or just like to click around with your mouse, you can do this by hand, but if you are you impatient (like me,

), you should use this program (
http://www.antp.be/software/renamer) and have it rename all the files like it suggests in this link (
http://forums.thesimsresource.com/showthread.php?t=337587&highlight=ant+renamer).
Once you have renamed the folder and all its contents, move it to the folder where your saved games are stored. If you have a basic installation, it would probably go into this folder:
C:\Documents and Settings\(User Name)\My Documents\EA Games\The Sims 2\NeighborhoodsWhen you start up your game again, you will have a new neighborhood at the end and it will be Pleasantview/Strangetown/Veronaville exactly like it was when you first installed the game.

Happy Simming!