Well, you can add on at concierge… I’m pretty sure if you arrive at your resort, you’ll be able to purchase extra MYW tickets.
You might want to go about calling ahead, though. That’s your best bet.
As for a cruise, it’s a lot more difficult to add a person. There can only be so many people on the ship… our friends had to purchase two rooms on a Disney cruise because they could only have so many people on the ship and had to book all rooms.