I have my iPod set to automatically sync certain playlists on a Mac at home. I sometimes download music from emusic.com or bleep.com at work, and I’d like to listen to them on my iPod on the way home. I could manage my iPod manually from both computers, but I don’t want to lose ratings and play counts. Using a seemingly undocumented feature, I found a solution:
1. On the work computer, connect your iPod and open iTunes. When told that the iPod is owned by another Mac, click Cancel.
2. Expand the iPod icon under devices and click on the iPod’s Music master playlist. The list of songs will be grayed out, because the iPod is managed by a different iTunes Library.
3. Drag your mp3s from the Finder to the iTunes song list window (not the sidebar), and they will be copied to your iPod anyway.
Disconnect your iPod and enjoy your new tunes. You can even add ratings, but beware — if you sync when you get home, the tracks will be deleted from your iPod unless you add them to iTunes first. To get around this, you’ll need to copy the mp3s to your iTunes library at home before you sync your iPod again. You have two options:
* From work, also add the tracks to the iPod in disk mode and add them to iTunes when you get home.
* Use Podworks or some similar utility to transfer them from the iPod to iTunes when you get home.
Just be sure to hold down Command and Option when you connect your iPod at home to prevent the initial automatic sync. Once the files are added to your iTunes Library, sync your iPod. The playcounts and ratings will even transfer!
From : http://www.macosxhints.com/article.php?story=20070202161325827