Since when can you import music from an iPod simply by copying the files?? The whole point about the iPod is the protection of the music... The only way I have copied from an iPod is by downloading software that will retrieve content from an iPod such as CopyPod. (This software is not sanctioned by Apple) Without that, you won't be able to see the music files, just a bunch of encrypted stuff that your file system won't understand...
And as for syncing with iTunes - the syncing is only one way. You can only push from iTunes to your iPod (unless you're using an iPhone/iTouch ) so syncing from iTunes to an iPod that has music not in your iTunes library, will simply remove that music.
So to get it off the Nano, download a copy of Copypod or something similar, retrieve the music, get it in to your iTunes library and maintain that so that any iPod only ever has a copy of the what's in your library.
HTH