Ipod Sync problem

Hello,

I synced more than 500 songs originally on my comp where I live, but I am visiting someone and I want to put more music on my ipod from their comp, they have itunes on their comp. But the only way I can sync music on my ipod from their comp is if I make a playlist with the song I want and when i sync it on, the other music I gone.
Sometimes when I put an ipod in, they have their own file on itunes, so I can drag music into it without having to make another playlist. But I can't get it again for this Ipod.
Someone please help!