As far as I know, the only way to store only part of your library in a seperate library folder is to add songs that are stored elsewhere to your library without the "Copy songs to iTunes music library when adding" preference selected. That way, they'll show up in your library (as long as you don't move them), but they'll only be stored in whatever folder you have them in.
You'd have to be a bit careful doing this, and it could be a hassle to move things that are already in the library elsewhere (the easiest way would probably be to remove the folders from the library folder manually, then delete the songs from iTunes, then drag the folders into iTunes to re-add the songs from the new location, making sure that preference item is unchecked).