I do not think you can do it in advance. I don't even think you can do it at your resort, you'll have to go to a park. We usually get to the park around 20min early and take care of it before going into the park. At MK there is usually someone by the boat docks, they only take cash but can do a length of stay.
After the first day you can bypass the payment area. Each park has a seperate stand where you get the stroller. You just show them your tickets, they date stamp it and you get your stroller. Very easy process.