If you see Show Contact Card instead of Create New Contact, you already have a card for that person.
Fill in the fields in the contact card. Be sure to enter any instant message accounts you want to be able to use to send the person messages.
If you’re showing buddies for all your active accounts in one list, and the list contains multiple entries for a single person, you can quickly add all that person’s accounts to a single contact card.
Command-click all the entries for the person in a View your buddy lists