What is the GH of the water? This is the most important of the three.
The better way would be to add Rift Lake salts, but you would need to buy a GH tester so you would know how much of the salts gives the GH needed by guppies. Both KH and pH would also increase, but as GH is the most important, get that right and let the other two settle at whatever they are.
You would have to work out how much of the Rift Lake salts to add to each bucket of water to set up the tank, and use the same amount per bucket for water changes. You would have to make sure there were some salts in the cupboard at all times as you could never to an emergency water change with plain water.
