We have the debit card...I just googled "disney debit card", and got to the disney page for both debit and credit. There's a tab for cardholder offers, and right now they only show the "kids stay/play/dine free" offer, but when you look at it you can see they give the phone number and code for it...
I imagine if it comes out, you'd call, and would probably need to pay the deposit using your debit card (that's the tricky part, isn't it?)...not sure about the balance.
However, Im' not really certain you could just *add* it to your existing reservation...they might have to rebook you entirely...might not be as easy as it seems.