Pretty sure that the dining plan can only be booked with a package including at least a one day park ticket and a rack rate room--unless you are a DVC member staying with DVC points. So, I doubt that discounted room rates would be able to "add" the dining plan. However, check with your EM maybe they can do something for you.
we were married march 21st at disney when we booked our guests rooms we were told that the dining plan was not an option with our room discounts. the dining plan is only available with a magic your way package including the room & tickets or as a dvc owner package option.