It will depend on if the dolphin swims are sold out or not. They only have so many dolphin swims available per day. So, if you're going during a busy part of the year, then you probably won't be able to if they are soldout. If you're going during a slow season, then you might luck out and you might be able to add on when you check in.
I guess, my answer is..I assume you could add it on, if they have swims still available.