There is no way to directly add money to a GC from say a credit card. However if the card you have is like a normal Dis GC with the 16 digit code and the 8 digit EAN number on the back then you will be able to add value to it. You will first need to buy a Dis GC, or cards, and then go to the Dis GC site to combine the money on the card you want to use. Each card can hold up to $1000 on it and they do not have to be "active" on the GC site to use them. It's recommended that you delete the card from the site. If someone gets your username and password they could empty the card. It helps to keep a list of the numbers from the card handy in case you lose the card so you can transfer the funds to another card.
Hope that helps.