People are quick to say that you can’t add a disc to a frame that isn’t designed for it. What would be the big differences? Disc specific chainstays or seatstays? They don’t exist. Disc specific fork blades exist. The others don’t. All the back end needs is a brace between seatstay and chainstay, along with the brake mount.
You’ll need a disc specific fork though. This would be a good time to have one made. Then you can convince the builder to mount the front brake where it belongs, on the front side of the right leg. That way you won’t need stupid crap like lawyer lips or thru axles.