The circuit on the link is a stand alone (small) stepper driver. The rotary switch can be bought separately. I have here one in my hand: Bourns, ECW1J-B24-AC0024. As it outputs AB pulses it should'nt be hard to implement. The real challenge is the circuit to enable hand wheel signals/disable PC signals vice versa, and eventually multiplex it over the axes. For the price off the switches I would'nt even bother with multiplexing, but use one for every axle.