How do I put the songs on my iPod to the library?

I downloaded a iTunes which was great and all except when I go to "sync" my iPod it wants to replace the songs in my library (there are none) with the songs on my iPod nano (which I have many many many songs). How can I put the songs already on my iPod into the library? Thanks.|||I believe you can only transfer purchased songs. My i-Pod will empty if I try to synch it to another computer.|||Under Edit, Preferences.... click the ipod tab and there should be an option to manually control your ipod as opposed to having it automatically sync. You should be able to do what you want after changing that setting.