Interesting question. As a new iPod and iTunes user, I was interested in this myself. So I gave it a shot with my iPod (nano 4th gen, synced to my windows XP computer iTunes), and added a couple mp4 videos from my son's windows vista computer (which had never seen my iPod before).
I did it successfully. Let me see if I can remember how I did it:
1. Before hooking up my iPod, I opened his iTunes. Go to the Edit menu, click Preferences. Click the Devices tab (way over on the right side). Click the box that says "Disable automatic syncing for iPhones and iPods."
2. Now I hooked up my iPod with the USB cable. It took a couple minutes to recognize it as new hardware, then it showed up as a device in iTunes. In the Summary tab of my iPod (in iTunes), I clicked "manually manage music and videos." It then (if I recall correctly) asked me if I wanted to delete the library on my iPod, and I clicked "no" or "cancel" or something similar. But the "manually manage" box is still checked.
3. Now I went to the Movies section of his iTunes Library. Found a couple short videos. Click and Dragged to my iPod. They added to my iPod successfully.
4. Eject from iTunes. Disconnect iPod. Before closing iTunes, open Edit, Preferences, Devices, and UNcheck "disable automatic..." (This is so my son isn't upset at me for messing his iTunes up, with him watching me as I do this.)
5. I go to the living room and watch the videos on my iPod with a little bit of self-congratulation. And my music files are still on my iPod.
6. Now I go back to my computer. Open iTunes. When I plug in my iPod, I see that it (on the Summary screen) shows that it is set to "manually manage..." Since I am done watching the video, and since I want my iTunes to sync automatically, I UNcheck "manually manage..." It gives me a warning message that I'm going to sync to my library. But I check OK, since I'm done watching the videos, and since I prefer to automatically sync with my podcasts, etc. (If I didn't mind doing it manually, or if I wanted to keep the movies on my iPod until I got around to watching them, I wouldn't have changed this setting.)
7. The iPod syncs to my iTunes, in the process it removes the videos (which is what I'd expect).
To those with more experience than I have: Is this right? Did I miss anything important?
