I've been trying to figure out how to do that as well.
I only have Macs at home.
I was given a pre-loaded Windows iPod Classic
I want to add songs from my Mac without losing what's already on the iPod.
The iPod shows up on my desktop, but adding music files manually via the finder doesn't work. New music doesn't display on the iPod. (Side note: I can add photos into the Notes folder, though, and those show up on the iPod.)
I tried doing the "manually manage music" setting iTines, but anytime I do anything with it in iTunes, iTunes says I have to reformat the iPod.
So far the only working option I've come across is to use an app to copy the iPod music from the iPod to my Mac, then put that batch of music into iTunes, reformat the iPod for Mac, and then sync all the old music plus my new music onto the iPod.