Yes you can add it to any reservation but you must do it at least 48 hours before check in. You just need to do it for all people on the reservation and for the entire length of the reservation.
We are doing this in September. We are staying 5 nights at Pop with the free dining and then moving to VWL for 2 nights and buying the deluxe dining. It's just long enough to try it out and go to some of the 2 credit restaurants. I don't think I would want the deluxe plan for a whole week. We have always been so full from the regular dining, I can't imagine eating 3 TS every day for a week and 2 SC!
thanks this is great news we are thinking about coming for the food and wine however I think the deluxe would be too much food for us especially with the food and wine going on I think we would be snacking all day