TOPIC: Add discount code after initial registration?

After someone registers, if you want them to edit their registration and input a discount code, can this be done and if so how? When I log back in to edit a registration, I can change everything but there is no discount code box to enter anything in. When you initially register, the option for a discount code is there, but it is not their when you edit afterwards.... and of course the discount code box on the admin side is only for displaying not inputing one.

I should add that this is for Pay Later/Status Unpaid.