let me think ......

You can't convert natural gas to propane because natural gas is methane and propane is what it says it is. Given that, there are Compressed Natural Gas cars, and if you get a natural gas compressor you can use it in a car.

car

The disadvantage of using natural gas in vehicles is the lack of fueling stations across the country and the cost to convert a car to natural gas.

A natural resource is just that... made by nature or more specifically by God the Creator.

A man made object like a car is "man-made" not nature or God-made.