Should be very easy. Disconnect the wire that's connected to the tip of the output jack and solder it instead to lug 3 of an audio taper potentiometer. (The pot will act as a voltage divider, so the resistance value doesn't really matter much. I'd recommend an A100K pot, as these are commonly used for this purpose.) Solder lug 1 to a grounded point in the pedal (the sleeve of either I/O jack will do nicely) and lug 2 to the tip of the output jack. Done.
BTW, all the JHJ-2 Fuzzfaces I've seen have a volume pot but no tone control (see image below). How is it that yours had a tone and no volume??
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