The dining cannot be added to a room only reservation. You would have to start all over by booking a package through Disney's Travel Company. Note that they also have a different resort inventory from Central Reservations, where room only is booked.
I had a room-only reservation for March, that I changed today to a package with the dining plan on it. I had a really goofy deal; not only did I change to a package, I changed resorts and dropped two six-day hoppers that I had on the original reservation. They just cancelled the original reservation and started from scratch. It was no biggie.
The only issue for me is that I already own two five-day hoppers, and with four of us staying at the resort, tickets were an issue. Each person on the reservation has to have the same ticket media, if I'm understanding it correctly.
What I did was add four one-day base tickets, so that I could add on the dining plan and I'll order two five day hoppers before we leave, or once we get there. Instead of five days at the parks, we now will have six. What torture.
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