I'm unaware of any companies that are developing something like this specifically, but you could replace the gas fill valve with an HPA valve to achieve what you're looking for. Something along the lines of this:
http://www.airsplat.com/cqb-russian-...tap-valve.html. You could just buy a second gas "mag" for the M870 and install this on it. It would pretty much allow you to swap between green gas and HPA pretty easily.
The only thing you would have to figure out is how to do this while keeping the stock intact, you may have to drill a hole into the stock or something to run the HPA line through.