How do I download music from Itunes without deleting old music?

I got a new computer with new music that I want to add to my Ipod. My sister tried it, and the new music replaced the old music and now they are gone. Is there a way that I can download my new music without deleting what I already have I my Ipod (which is not in my itunes) ?

Thanks!How do I download music from Itunes without deleting old music?
When you plug in your ipod, quickly stop the sync. On the summary page, uncheck an option that says only sync checked playlists. Threats probably why its causing it.