You may need to get a little clever with this. However yes, you can do it. It all depends on how often you will be changing their balance.. if not at all then this module could work,
This module: http://www.magentocommerce.com/magento-connect/subscriptions-and-recurring-payments.html
You could create a reoccurring payment on a hidden “credit product”.
Yeah, this is hacking Magento a bit, but a custom solution would cost a bit of money.