Hi there -
If you set the iPod to synchronize with the iTunes music library, it will most likely wipe the content already present on the iPod and keep a 1:1 parity with what is on your iTunes.
My suggestion would be to install iTunes on the computer, and to manually add the same files you had added to the iPod directly onto the iTunes library prior to synchronizing.
If you don't have all those files, connect the iPod the same way you do now, and copy all its contents locally. Then, install iTunes and place all those items (you can drag the whole directory onto an open iTunes window) and that way you'll have everything currently on your iPod copied onto you iTunes music library before getting it to sync with your iPod for future use.
Hope this helps!
Itay.