Ipod is synced to another library. Can I add new songs without losing existing ones?

You should find that if you connect your iPod to the library it's synced to, you can check a box (which I think is on the summary page) which will allow you to manually manage content/music on your iPod. If this box is checked you can connect to any library and drag content over to the iPod (doesn't require syncing). It's a while since I've done this so it could've changed, but this is the way it was.