You can't add new roads to an existing neighborhood..but you can get the "Hood Replacement" program at MTS2 by Mootilda. This program lets you copy roads, terrain and other things from one hood to another.
http://linna.modthesims2.com/browse.php?f=139&u=589252One warning, though: if you have houses and families you want to save, do that before you replace any details.
I've done it a few times, and it works great.