Originally Posted by Ben Thornton
The easiest way is to sync music files from iPod to Mac
using a USB stick or a drive. If you have to use the iPod, there is software available for the Mac which will allow you to access the files on it, and therefore treat it as a drive and copy them off. Then you'd load them into iTunes, and you can sync as normal. If these other tracks were purchased in iTunes, you should be able to download them to the other Mac without repurchase, so a copy wouldn't be needed.
Select your iPod in iTunes, click the iPod options button, and change the synchronization management mode to Manual. iTunes won't auto-synchronize anymore, but you can now control what iTunes does when it syncs your library with your iPod.