Sync is only oneway, from PC to your device. Unless you have the music on your PC, iTunes is going to wipe out what you have on your device if you are syncing to a new library.
You can only transfer Purchased music over to Itunes on your PC.
iTunes Store: Transferring purchases from your iOS device or iPod to a computer
As for you own music, you may have to use a third party software. A good Free one is called Sharepod which you can download from Download.com here: