Don't know if you have seen this article, but possibly it may help you to do what you are looking for:
Since the article is not having you move the copy of music on your iPod into iTunes directly, I'm thinking it may serve your purposes....
Hope it helps!
Hi Gail, thanks a lot for the quick response! Unfortunately that's not what I'm trying to do - that tip is essentially how to use your iPod as an external HD.
What I am looking for is a way to take the music already on my iPod (which is not all on my computer) and back it up whereby I can later transfer it onto a different, blank iPod in a way that preserves my dozens of playlists with hundreds of songs each without me manually recreating them.