Is it possible to set up different sources in Front Row?

Yes it is, but they will only appear in the Movies folder. Here's what to do for Movies:

1. Go to your NAS drive, find the directory or file(s) you want to be able to access from Front Row.

2. Right click on the directory or file(s), and click "Make Alias". This will place an alias in the same place as the directory or file(s).

3. Drag the alias(es) over to your Movies folder on your Macintosh HD.

4. Open Front Row, navigate to Movies, and you should see the directories or files(s) there.

This works on both Tiger and Leopard.

For Music, since it calls directly from iTunes, you might want to try doing the same thing, except dragging the aliases to the Music folder. I have not tested this though.