Group the door with the knob then add the entity afterwards. Then knob and door will move together.
btw there are some built-in-doors with animated knobs that look nice. If your satisfied with the looks of them use them instead of creating a door from cratch.
If you make a prop_door_rotating entity and set it's world model to a model's who's name is something like door1_left in the c17 directory you can set the hardware type to lever for a doorknob or bar for a pushbar. It won't show up in hammer but will in game.