Purchasing additional storage will not use account credit anyway. It gets directly charged to your credit card, and you must use a credit card since it is an annual subscription service you are purchasing, not a one time payment option. They insist on a credit card so they have a billable account for the automatic annual subscription fee. At least it was that way when I did it.
Sorry - http://support.apple.com/kb/HT4874 either I did not have enough credit at the time or something has changed since I did it? Maybe call Apple and do it over the phone (explain why you want to do it that way)?