A "heatpump" is an AC with a reversing valve. Yes you can just get a reversing valve and have some hack cut and solder it in for you. I'd assume any normal contractor would charge $1000+ for that job though. It would take multiple hours for a tech when instead he could make multiple house calls in that same time (and possible making more money doing house calls). So that is why you won't find someone to do it. It make no sense. Right now on ebay you can get a "Goodman 4 Ton 14 SEER Heat Pump" for $1800 delivered to your doorstep.

Oh and I guess another thing a heatpump has that an AC doesn't is a defrost controller board. You'd need one of those too.