iTunes is strictly one way computer to iPod. Anything on the iPod that is not in your iTunes is lost when you sync it. There are 3rd party programs that allow you to backup your iPod to iTunes, which is what I use to sync my iBook iTunes with my iMac iTunes; it's all music from CD and I've added the lyrics and artwork. However, you can easily slip into copyright issues if you're trying to get music from someone else's iPod, which is partly why the iPod/iTunes link is officially one way only iTunes to iPod.