I have an iPod my friend let me borrow and I want to upload my music into his. How can I do that without erasing his music he already has on there?

You can't really do that. You can either keep the iPod as is or update it with your songs only and delete his.

Or you can get the songs off his iPod, make a separate playlist and upload that as well along with your songs. Of course, this way would be more complicated.

I think there's a program which enables you to do that, I forgot what program though..

1. Open iTunes after connecting YOUR iPod
2. Change preferences to "manually add music & videos" "apply"
3. Disconnect your iPod
4. Connect his iPod
5. Drag and drop into his iPod

I have the new version of iTunes, so that's how it'll work for me, but before, I know the outdated versions, you had to go to "Edit" "Prefences" and then on one of the tabs, you'll find the "manually add music" option.

^If that doesn't work, try this program:

http://www.ephpod.com/download.shtml

you could update from his computer only.

^That's not true; I've borrowed my friend's iPod before, and have transferred files to and from it, and I don't even own one myself.