Plug in your iPod. When iTunes opens, it should ask you if you want to update your iPod with the songs from this computer's library--You Must Click NO! If you don't, you will erase all the songs from your iPod. Your iPod should be mounted on the desktop.
Drag the Addressbook v-card from your iPod's Contacts folder into your hard drive where you can access it from your other user login. Try the Shared folder in your Users folder.
Unmount your iPod, then log back in as your other user.
Open up Addressbook and add the contacts.
I just tried this on my wife's nano which is linked to her PowerBook, and it worked great.