I would like to be able to do this as well, though after 4 years it looks like you can't do it.
Every so often we sell someone a GIFT CARD in one of our stores and they think they can use it on our website as well (they can't since the two system aren't tied together). No problem issuing them an on-line gift card, but it is a problem with them still having the physical card out there saying there's a $100 in value on it.
I realize I can 'pretend' they bought something at the register to adjust it down to zero, but then I end up with a fake transaction on the computer throwing off 'real sales' and real inventory.