If you have a Disney account and have booked with them, you can make a payment online any time as long as it's at least $20. Log into your account, put in your reservation number (if you haven't already) you'll see a link for "My Reservations & Tickets" - when you click in there, you'll then see "make a payment" and the rest is self-explanatory.
You can probably call 407-934-7639 and put money on it as well - if you have your reservation number, there really isn't any reason I could see that you wouldn't be able to.
This is probably one of my favorite things about booking directly with Disney. It's very similar to booking your dining reservations online with them if you've ever done that.
I literally "throw" money on it whenever I can. It makes budgeting so much easier and makes it much easier to plan ahead.