This is the easiest way to do it. I'm not sure they can do it if you call. It might depend on the ticket (package, etc.). Any ticket window or Guest Relations location can do it though. If you only have a one day ticket, make sure to add days before the end of the day of use. You do not have to do it before you use it to enter a park though, so you can do it on your way out or whatever works best for you, but you can't extend it once that day is done.