Vanity Coupon Codes

I have setting to auto enroll new accounts into the affiliate program. I would like to allow the customer/affiliate to be able to change the coupon code to whatever they like, to update their personalized coupon code as they please.

It currently makes their coupon code the first part of their email address before the @. Which is ok, but would be nice to allow them to change it to whatever, like joeblow20, since we give them a 20% coupon to give out.