"Room-only discount" is a bit of a misnomer these days. The discount can be used on both room-only and package reservations; it's called a "room-only" discount because it
can be applied to a room-only booking, whereas a package discount can only be applied to packages.
You can book a package with a room-only discount. It has to have either tickets or a dining plan to make it a package. So yes, you can book a package with a room-only discount and dining plan, and you aren't required to add any tickets at all. You can't book a ticketless package online, though; you'd have to do it on the phone or have a
travel agent do it.
One thing you can't do is book a room-only reservation and then later add tickets or dining to it. That would require a cancel and rebook, because room-only reservations and package reservations are totally different as far as Disney's systems are concerned.
In practice, if you ask a Disney CM or a travel agent to add tickets or dining to a room-only, they'll just quietly rebook it for you as a package, if there's availability. But behind the scenes, it's not a change, it's a new reservation.