well when you "sync" something, it basically means delete any existing content and replace it with the content that you want to sync it with. so click on your ipod icon. click on summary scroll down, and under options you should be able to select the option to manually manage your music and videos, so your ipod will not ever sync therefore not erasing any thing that is already existing.
instead of clicking sync, just click autofill and all your songs will be put onto your ipod without anything being erased.
hope that this helped :)