I’m sure there’s applications where that’s true, but then you’re essentially talking about having a gas furnace plus a heat pump, so you’re installation cost is close to the sum of both systems. Energy rates vary by region, but around here electricity is about 7 times the cost of gas, so a heat pump running at a coefficient of performance around 3 would still cost twice as much to run as a natural gas furnace, it would be cheaper to just turn off the heat pump altogether and only use that “supplementary” heat.