how do i download songs from another itunes library without deleting the ones already on my ipod?

ok so i want to download all of the old songs from my other library but it says i have to erase and sync to get my old song on my ipod but i want to keep my new songs from my new library still on my ipod... is that possible? if so, how do i do it... i have already done the Itunes matches thing but to put that on my ipod i have to delete all of my songs on my ipod too.