It can be done, but unless it's a link card (since our state's cards are crap and don't scan) or a team member's card, we won't do it, because it could be fraudulent.
But if you insist, just select credit or debit in the payment screen, type in the amount you want (or hit the k1 key for full amount) and then it will give you and option to type in the number. If it's not a Target card, I know the credit card will require a supervisor override. Not sure about the debit.