Usually all you have to do is drag it into a iTunes playlist and the next time your iPod synchronises the file should end up on you iPod.
If this is not working try putting the files in a folder that is not shared with Limewire and have a go at dragging it into an iTunes playlist from there.
Be careful that you are dragging it into a playlist and not just the iTunes Library, otherwise it will only be in your library and not transfer to your iPod at the next sychronisation. In order for a song to appear and play in your iPod you have to put the files into an iTunes playlist, not just the iTunes library.
I suppose you know all this already but I just thought I'd mention it anyway.
Songs in your iTunes library will not transfer at synchronisation wheras songs in a Playlist will transfer into your iPod at synchronisation.