If you booked a package (room and tickets) then yes, you can add the Dining Plan up to 3 days before arrival. If you booked a room-only reservation then that ressie would need to be converted to a package with tickets and there is no guarantee that you will keep the same room category that you have booked since they will have to cancel that ressie and book a new one. Although some have had success doing this by asking to speak with Guest Services when making the change.