you'd have to be an experienced pipe maker to adjust a set of pipes for lower pressure/volume and still make them play in tune - in fact I'm not sure it would be physically possible without replacing most of the instrument as the length & bore of the drones and chanter and the positioning of the finger holes all come into play.

All the pipers I know have two volumes - on or off. (mostly on English pipes which are not as loud and raucous as Highland Pipes but can still be pretty loud).