"You can add users who aren't in your domain to the shared contacts list using the Shared Contacts API script. The Shared Contacts API will also allow you to enter in additional details like address, phone number and more to domain contacts. These additional details will only appear when shared contacts are viewed in the Contacts Service. The Shared Contacts API is supported by our API team."
http://www.google.com/support/a/bin/answer.py?answer=47926