When we booked our cruise, our travel agent was having difficulties adding the AI option. He ended up booking it as a cruise only fare, then he called X after to have the AI option added. It went through as two separate charges on our credit card. So i think it may be able to add it after.
First time X cruiser here. Made a booking mid-November for an inside stateroom for a Dec 31st sailing during the Black Friday sale. Decided to try my luck and made the minimum bid for a veranda ($100pp) Crickets for a couple of weeks and just saw today that I was successful. Good luck everyone!