How can I get my new music onto my iPod without erasing the music that is already existing on it?

As we all know that, iTunes is a great tool that lets you sync music and videos on your iPod and iPhone to iTunes (and iTunes 0.9) easily, however, you are not allowed to export your iPod and iPhone contents back to your Mac via iTunes. It will make you headache when sync iTunes 9 to iPod, and whats' more, if you delete your iPod music by accident, firmware update, or send device to mend, in these cases, you need an iPod to iTunes Sync to help you.

Free download Wondershare iPod and iTune Sync for Mac (Intel only).

Step 1, launch TransPod, and then connect your iPod with Mac.

Step 2, Choose files you would like to transfer to iTunes from “Movies” or “Music” folder. Here you could also copy iTunes Library including "Movies", "Music", "Photos", "Playlists" to iPod.

Step 3, Either drag-drop the files to iTunes, or click “iPod to iTunes” button to export the files to iTunes.

As you can see, with only a few clicks, you could move and export your iPod music and song to iTunes on Mac OS X.
How to transfer videos and songs from one iPod to iPod on Mac OS X