You have to purchase the room and tickets from Disney in the form of a package reservation to be able to add a dining plan. If you don't have a package, no plan. You don't have to buy everything as a package to start with - you could buy the room and then add the tickets and dining later, converting the reservation to a package; you can also buy the room and tickets as a package and add the dining plan later (must be done at least 3 days before arrival) - but you can't book a room only reservation, purchase your tickets from a discount broker, and then add a dining plan. To get the plan you would still have to purchase at least a 1-day ticket as part of a package reservation.
The exceptions are annual passholders and those eligible for the military discount (who can purchase ticketless packages) and DVC members (who do not purchase packages but may add a
DDP to their DVC reservations).