howdy (yes, i ask lots of questions while learning a tool ) ..
On the Customer Settings | Customer Form Fields page, there is a sub header that states:
"You can create and manage the customer form fields available during registration below."
As far as i can tell, you cannot "create" any new attributes. You can enable them for display. This is disappointing as i want to add one that states: __ I wish to be an affiliate .. and this will reduce admin overhead in the task of organizing such folks.