How would one go about converting a heat pump system to accept an alternate heat method? The outdoor condenser/compressor/fan unit on this house died, so I'd like to use the rest of the working parts of the system to heat the house this coming winter. I'd like to install a gas fired heat exchanger (radiator) in front of the recirculating fan thereby taking advantage all remaining components that still work. What additional parts would be required after 1) a gas boiler 2) the heat exchanger (radiator) 3) water temperature sensing thermostat 4) heating water recirculating pump and 5) necessary plumbing to connect the two together and tap into the water supply for make-up water?
I've seen something like this done using wood fired water coming from an outside wood furnace to a 2 foot by 2 foot radiator placed in the path of the indoor forced air. Both of these houses are in Virginia where the heat pump concept still works during mild winters, so I'm directing my question to home owners in colder climates where the expense of electrically (emergency) heat has proven itself uneconomical.

You would spend many thousands of $$$ to accomplish little. You would lose AC. Just replace the condensing unit and coil with a higher efficiency unit designed for colder weather or go with a forced air gas package.