Everytime i try to sync new music from limewire to itunes to my itouch it makes me delete my old music for new?

it says transfer purchases or erase and sync. if i click erase and sync then it deletes all my old songs for new ones. i use my itouch on same computer so why does it say it can only be synced one at a time. please help im tired of not puting new songs or adding them and search all my old ones.