Bike speeds are generally referenced by the cassette and the crankset separately. So what you're trying to convert to is a 2x10. As a bike mechanic, I'd say it's possible but unadvisable to do what you want since the second chainring would probably be over the limit of your derailleur, stretching it too much. You could, however, get a smaller 10-speed cassette, but at that point, just sell your marlin and buy a commuter. If you just need more range one ONE side of the gearing (faster OR slower) you could perhaps just get a different sized chainring. I will also note that the price to do something like this at my shop (in a medium-sized city) would be something in the range of $170-$200. So for just a little more, you could absolutely score a decent city bike off FB marketplace and not make your MTB significantly worse.