You can modify your pistol any way you want short of turning it into an NFA item or an assault weapon. Will Kahr sell a non-MA slide to a MA resident? Only Kahr can answer that. It's not illegal but may be against their policies.
This. That and the ridiculous safety that is likely never used. I am amused when I see the classifieds jacking up a PM9 price with the oh so exclusive "free state" tag! Anyways free market. I'm with drgrant, pay the premium (unless you know how not to) for a "free state" version or learn to put up with marginal "features" and use the extra money to buy ammo