How can I use a generic doorknob as a capacitive touch sensor?
You could try and use the entire doorknob as a single-node capacitive sensor: 

The electrode for such uses need not be ferrous or cuprous, as long as it is conductive. 

The mechanism involves sensing capacitance changes between the electrode (doorknob) and any earthed conductive body (the human). No second electrode is required; Instead the other pole of the capacitive sense circuit is connected to ground.

For bonding a wire to the metal of the doorknob back plate, a simple method favored by DIYers is the MG Chemicals Silver Conductive Epoxy. It comes in convenient little syringes:




Clean the back-plate carefully first with alcohol or acetone, then with a fine emery paper. 
Also scrape any oxide off the wire. 
Epoxy the wire and a bit of its insulator onto the back-plate: The glued-in insulator works as a strain buffer so the wire won't break off too easily. 
Try and get as much wire in contact with the doorknob as possible, since the epoxy isn't quite as conductive as a good solder junction. 
Avoid getting any of the stuff onto your fingers. 
Use a hair-dryer set to minimum airflow, to warm up the epoxy - this cures it rapidly and maximizes its conductivity as well.


That should sort it out.