Create a contact card using Messages on Mac
You can create a contact card for a friend right from the Messages app. After you create the contact card, you can easily send that person a message by typing their name in the To field of the Messages window, or clicking the Add button next to the To field and choosing them from the list.
In the Messages app on your Mac, select a conversation, then choose Buddies > Create New Contact.
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.