I tried & if you've already paid, no it won't let you online. You can only use it to pay for seat assignments - that is IF you haven't already paid for them with your original booking. If you chose no seat assignments with your booking, you can go back & add that option at a later time, and use a coupon code for that. You might be able to pay for baggage fees if they haven't been paid for, but I'm not sure on that one, we only travel with carry-ons.
A friend called once because she forgot to use her coupon code online when booking, and they would not allow to her apply it after-the-fact.
If you find a way to get around this let us know please!