Deedee,
I'm in the same situation. I bought my park tickets online months ago. I just called the online ticket customer service number(407-224-7840) and told them I would like to add the "Stay and Scream" option to all three tickets for October 12. They added it on for me(for a low price of $24.95 each

) and now I'm all set. Give them a call!
P.S. I then went back online and purchased 3 express passes for the same night! Can't wait to go!!!!!!!!