There appears to be no way to combine gift cards and you cannot add additional funds to an existing gift card. Some people have noted a hack to this in that you can purchase a dummy booking with them and then cancel which results in the gift card being added to your Southwest account as Travel Funds YMMV.
Southwest gift cards – A maximum of three Southwest gift cards can be applied per Passenger on each reservation. If your purchase exceeds the amount available on the three Southwest gift cards, you will only be allowed to use two Southwest gift cards and another form of payment will be required for the difference.
As noted above if your booking exceeds the value of your gift cards then you can essentially only use two gift cards and then a third form of payment that will cover the remaining balance.
