"KH is 0 and GH is around 180"
You do not need to add calcium to this tank. (GH)
Add carbonates. (KH)
Potassium bicarbonate is a source of carbonates that does not add more sodium to the water.
You could just add baking soda. Go for it.
You will be doing a water change before adding fish, so I would not worry about the sodium level while cycling.