Yes, HK could do the same thing that he did. But that would take engineering time and money. The questions isn't can they do the same thing, but rather should they and at what cost?
HK didn't give him a technical package with a 3D CAD CAM model. HK didn't give him the instructions for CNC machines to mill out the parts. HK doesn't have any of that because they never had it -- the P7 manufacturing ended long before that. So to tool up for the gun, HK would have to take their blueprints and/or scan their existing guns, create the models and CNC instructions. They would then have to decide what to change, build prototypes, test them, refine them, etc. All of that takes engineering time. Why should HK spend time and money to do that? It's a niche market.