If the bulk of your Hatsan's noise is spring buzz, then a properly fitted spring guide and other measures such as polishing the ends of the spring are going to be much more effective than a moderator. You can better distinguish muzzle pop from spring buzz and piston clunk by standing off a safe distance to the side, while someone else shoots your Hatsan..
Timothy,If you have a DonnyFL and simply want an adaptor, such a thing can also be custom designed and printed. I envision a threaded metal tube, threaded into a plastic mount that slips onto the barrel for a sufficient length. But, you need to show an interest in your own thread, before anyone else will.