It's best to mix Aquadur with some tank water in a jug and stir until it dissolves before adding to the tank. Seachem make a liquid product (Seachem Replenish), but it only deals with gH, not kH. You could use that for gH and sodium bicarbonate for kH as it dissolves much more readily (but still best to dissolve before adding to tank). As a guide, 1 level teaspoonful of sodium bicarbonate will raise the kH of 50L of water by approximately 4 degrees.
How much you need to raise the gH and kH (if at all) depends on what you're keeping.