I havent done any “grabbing” of this manner yet…but if i was going to. Id prob first set some how that the system knows the motion controller is in contact with the door knob…then use a button maybe the trigger to grab the knob and regardless of player movement if the trigger is still pulled you would still have a hold of the door. Kind of like job simulator does with grabbing.
A friend of mine who works with Unity wrote this blog about some of his thoughts developing hand interaction.
Might be interesting to you since he is talking pretty generally so engine doesn’t matter.
I also read these more Unity and physics interaction focused blog posts