| First, you must make sure that your iPod doesn't automatically sync with the computer. If it does, it'll wipe out all the music that isn't in the library. On the new iTunes, (7.0+) when you plug in your iPod, you have the "home page" with all the preferences you can change. When this pops up, hit "Manually load iPod" or "Don't sync" or... whatever it says (It's the first option on the list for the music section). Then hit apply in the lower right hand corner.
As far as adding someone else's library, that depends on what kind of music they have. If they purchased it from iTunes, your computer won't be able to play it because your computer isn't authorized to play it (you might need to log into your friend's iTunes account to authorize it). If it's uploaded from the net or CD's or anything not purchased, it will work fine on your own.
If you're uploading from your friend's computer, I'm not quite sure how everything is set up as far as if your iPod will sync before you get the option to not sync or ... whatever. | 
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