I assume you mean Adventure Island the waterpark next to Busch Gardens,I've never seen the option of adding it on to the flex ticket but something tells me there should be a way to add it on maybe on a separate ticket by visiting guest services at Busch Gardens or Sea World.It might be a one day ticket on it's own but I can't imagine they wouldn't want to take your money.Many times when I'm at Busch Gardens they have banners saying to add on Adventure Island for a deep discount.It is a very nice waterpark it's just not as big a draw to tourists as the Orlando waterparks,many tourists come over for a day for Busch Gardens but probably only a percentage go to Adventure Island as well,it's also not open year round like the Orlando waterparks,if it was in Orlando it surely would be open year round.If you don't have an option to add it on to the flex ticket I would look into seeing what kind of discount they might give you when visiting Busch or even Sea World.