You may know this already, but in case you don't: the iPod and iTunes system was specifically set up to go one way only, from iTunes to iPod, and not the other way around. This was an anti-piracy concession on Apple's part to prevent everybody from spreading their music around to other people's computers.
As the previous replies indicate though, there is third party software that allows you to do it. Just don't look for a way to do it within iTunes, because there isn't one.
Also, if you succeed in getting the music off the iPod, if the music files were purchased from the iTunes Music Store and are 'protected AAC', they won't play on the second computer unless you authorize the computer in your iTunes account to play the files. I believe you can authorize up to 5 computers to play your files. Not sure if you can import protected files into iMovie.