These changes do not get the low 20 gear inches you want. But 26 gear inches should be plenty low enough for Rockies and Sierras mountains. These changes are simple and easy to make. Not terribly costly. Putting a triple crankset onto the bike requires getting a new shifter for the front derailleur. And the crankset and new front derailleur. The ease of just putting on a new inner chainring and new cassette might override everything else.
The idea suggested by staehpj1 is also very good. Shimano and maybe SRAM too make mountain bike cranksets with two chainrings. So you can buy an off the shelf solution. Not cheap. $206. 26 teeth appears to be the smallest inner chainring from the factory. But the inner is a 64mm bcd. So a 22 tooth inner ring will fit. Extra $16. See links below.
Current low of 36x32 is 30 gear inches. My new 33 ring and 11-34 casssette gets you low of 33x34 and 26 gear inches. Cost of $130. New 2x10 Shimano crankset and new inner 22 ring gets you low of 22x32 and 18 gear inches. Cost of $230.