I added the dining plan just today...
I already had several reservations made including HDD. I had them refund the money I had paid and they changed the reservation to indicate that I'm on the plan. I had about 4 other reservations and she said it was up to me if I wanted her to go in and put that I was on the dining plan. She told me that it would probably be a little less hassle if I added it. She was able to pull up all my ressies by my phone number and do it very quickly.
I think it's just up to you.