Sounds like it could be a permissions thing. The default location for iTunes music is in your user folder, which is off limits to anyone else (any other user). Maybe this is preventing them from being shared via Limewire.

Try moving your iTunes Library folder into your Public folder or into the Users/Shared folder. Then open iTunes, and in the Preferences, relink to your iTunes library folder at the location it is now.

Then try seeing if you can share them.

Or try copying/moving them into the same folder as the ones you've downloaded.

Hope this helps....