[Bellardia]

You go to:
User Profile Fields > Add New Profile Field
to add another, it can be called by $post variable.
It is also stored in the $userinfo variable but is not parsed inside templates, although it can be accessed inside plugins by using the vbulletin class $vbulletin->userinfo['tablename'].

The $thread array isn't stored globally, so adding fields to it won't include their contents globally.