Compressor regulator

Is it that the regulator can't reduce the pressure or that the gauge only goes down to ~20 PSI. i.e. could you just change the gauge? FWIW though I'd be fairly certain you could remove the old regulator and install a new one.

The regulator only goes down to 20 psi and it is a square box built into the compressor.

Regulators can generally be added to the output of a compressor with regulator.  You just need to get the right pipe fittings.  You can stack regulator and filter/dryer, for instance, plus things like disconnects, tees, etc.

As Don said above, "You just need to get the right pipe fittings".

My wife and I share a 3hp industrial compressor with a 10 gal. tank. It's down in the basement in her workroom. She uses high pressure for her power staplers. I teed another line off the compressor and fed it up to my workroom in a spare bedroom upstairs. I bought a regulator/filter at Home Depot that gives good control down to 0 pounds and installed it on the side of my spraybooth. I can spray at any pressure I like up to the setting she has downstairs, which is much higher than I would ever use.   We can both use the compressor at the same time with no problem.

Jim