I see two ways that you could do it.
1 is to just have the power going through a switch on your dash or console or wherever going to the overdrive pin and another wire and switch going to the lockup pin on the side of the transmission.
That way you can physically Lock it or overdrive it when you want to.
2 Is to use pressure switches to control when it shifts into over and lu. With a switch on the console or dash to shut them off.
You would want to make sure it only works in 3rd gear either way.
