Dosing EI does not automatically add components like Calcium, although you could include Calcium Chloride (CaCl2) or Calcium Sulphate (CaSO4). You could also use GH booster. As far as I can recall anything in these mineralizers can be reproduced so it's not that big of a deal. It's easier and cheaper though to adjust the RO with tap instead of buying more things to remineralize it with (unless your tap is very soft). An advantage of using the commercial products is that the bottle tells you exactly how much product to add to achieve a particular hardness whereas the homegrown stuff has be figured out. This is an advantage if you have a specific kH/GH target for fish, but otherwise there is no need for accuracy, a pinch of this or that will do.
Cheers,
Cheers,