Can a converter box be used with cable TV instead of leasing their set top box?

Maybe. It really depends on the cable company and what they're sending out. Some channels may not need to use the conversion system in the cable box and the converter box might decode some of them. But, keep in mind that this isn't always true, since the boxes are controlled at the cable company and they have the options of using the microprocessor inside the box to control your subscription level. Try it and see. I think you'll be disappointed, though.