Getting iTune to add remote items to library
you can try  mt-daapd (firefly server) to serve media files to your 
iTunes from your ubuntu machine. 
Setup it on your server* and then add in Itunes Preferences->Advanced where your library is..

*this is a tutorial on how to install mt-daapd on your ubuntu server

http://all-tech-thoughts.blogspot.gr/2010/09/how-to-setup-firefly-mt-daapd-on-ubuntu.html

Also bear in mind sometimes for some versions of itunes mt-daapd compatibility breaks.
Updating to current itunes version usually helps...