Can you wire a standard on and off switch into an already existing circuit so that the off position completes the old circuit and the on position routes a new circuit?

Answer 1

A standard switch opens the circuit when in the off position, so the answer to your question is no. That said there is a way that it can be done by changing the switch to a single pole double throw switch. The "hot" will come into the switch on the common terminal. The old circuit connects to the top switch handle up terminal. The new circuit connects to the terminal in the handle down position. This setup will leave one of the circuits on all of the time. To over come this situation the switches can be installed in a double gang box. A standard on off switch will control the power to the "hot " that comes into the SPDT switch.