You can actually add a dining plan to a RO reservation. You just need to call and do it. Can't do it online.
You can add a dining plan onto a ticketless package*...a basic plan. You can't add a dining plan onto a room only reservation. They are separate systems. If you call, they have to book a new package (with a new deposit) to have a dining plan and then cancel the room only reservation (and refund your deposit within 1-2 billing cycles).
* frequently you can add a DP onto a ticketless package, but not for all offers. Gotta check the small print.