With Mountain Lion providing integration it is possible to have a contact's twitter username listed as a part of the contact. I don't see any way to manually do this before an account has already been added.
There seems to be two separate social media lists available. The first looks like this.
As you ca see it provides a list of quite a few networks, but no twitter. Once a twitter account has been added a second list seems to become available though, and lists different networks
This second list doesn't become available until an account has already been added. Which causes issues if none of your contact's details match with their twitter account, but you know their account and would like to add it.
Anyone know of a way around this?