Custom Gift Certificate code ?

We had a radio giveaway of $100 free in our online store. We gave out discount codes of 'Winner 1' - 'Winner 30' (there were 30 winners.

Unfortunately, the discount codes don't cover shipping so we were hoping to make the Discount Codes (Winner 1 - Winner 30) so that the Winners could shop with shipping covered by the code.

Is it possible to write custom Gift Certificate codes? or is it only auto-generated? Am I correct that Discount codes still don't cover a discount AND free shipping?

Thanks For your help!