It would probably be expensive, but it might help in the case of a shortage.
Some folks have only a weeks supply of propane in some of the bitterly cold areas, so something should be done...
Not without building special reactors that would take a few years. And it would be at least 4~40 times the price. You get some byproduct propane from the crude oil making the gasoline & diesel. Taking those finished products to crack them into propane would be very expensive.
The vast majority of our propane supply is propane already existing in the natural gas supply. This is not propane converted from natural gas. It is propane separated out (as if a filter) from the natural gas which is mostly methane.
It is not done with a filter, but rather with pressure and temperature changes that cause liquid droplets to coalesces out of the gas. But if you don't understand that process, it is close enough to think of it as a filter. Molecules are not being changed, just separated.