I see that you've already added it, but I did want to add this disclaimer to any who read this thread later who may not know.
No, you do not have to have park tickets to add the dining plan to a room only reservation. As stated above, it does become a "ticketless" package, and all package rules apply. And, as you found out, you are able to retain your room-only discount.
That said, you DO have to have tickets to dine in restaurants inside the theme parks. It gets asked here more often than one would think.