I did it, i added some mushroom compost and lots of vermiculite to make it more airy. Promix is like a sponge, so you need to make it less dense when it takes in the water. For one bale of 3.8cf promix, i mixed 1 bag black kow, 1 bag mushroom, and 3cf vermiculite.
UPDATE: i also added some epsoma tomato-tone. Potawie recommends adding compost to Promix so that it is more natural and helps the build the soil's health and beneficial microorganisms.