I had the same problem once. Maybe there is an easy solution to the problem, but I saw it as a challenge to control it myself.
Have a look at my published profile "P3D - Cessna C208B Carenado" (by CBird), Radio Panel, set Upper Selector to XPDR, Left Window = Barometric pressure.
When you long press the Upper ACT/STBY button, then in the Left Window the barometric pressure switches from in.hg (American) to hPa (European) and back. I had to create two local variables (BAROMETER_SET_TO_HPA and BAROMETER_SET_ACTIVE) to make it all work, but in the end it works really nice. It is not aircraft dependent.
If you put the Lower Selector to XPDR, then a long press of the Lower ACT/STBY button resets the barometric pressure to Standard Pressure (29.92 in.hg or 1013 HPa), which is easy when passing Transition Altitude in climb.