iPods are designed to keep it a one-way route to prevent copyright infringement, so you can't do what you want with iTunes. You've got to options:
1. Connect and open your iPod like a flash drive, and enable/show hidden folders. There's a folder called something like Ipod_Control, can't remember properly as I don't have mine with me. In it you'll find a folder with all your music; the files will have awkward names like QR4W.mp3 but if you add them to your iPhone it should show up fine.
2. Easy way: Try downloading and using SharePod (
http://www.getsharepod.com). I've never used it so I can't guarantee anything. Be sure to use it's backup feature. This is also useful if you want to restore your library on your computer in an organised way, unlike the mess they're stored in on your iPod.