If you are flying an N-reg aircraft with a mechanical altimeter in the UK/Europe then you just need to carry a small conversion chart to get you from HPa to inHg.
Or you could always ask ATC, as one pilot (allegedly) once did::
"N12345 descend altitude 3,000ft QNH 1022"
"Could I have that in inches please Ma'am?"
"N12345 descend altitude 36,000 inches QNH 1022"