I have a junction box in my crawl space where a wire coming from the electrical box is joined with a wire going to other areas of the house and a wire going to the outside. Can I install a light switch on this box?

Sure, as long as the box has sufficient room inside to accept the switch, and you use the proper switch cover for that box.

What size box, and provide details of the size and quantity of wires inside the box.

A photo would be very helpful.