Yes, the white precipitate most likely is calcium carbonate.
Calcium carbonate is basically the same stuff that coral 'rock' is made of, so it won't cause any harm to the tank even if it is present in large amounts. It is, however, possible that calcium carbonate crystals could grow (precipitate) quicker onto pre-existing crystals that when forming new crystals, and if you keep your calcium and alkalinity levels abnormally high you might want to regularly remove any precipitate, but under normal conditions I would not bother...
I dose my BiCarb, Baking soda and Kalk via the auto top up.
I hope that you are not dosing all three additives into the auto-topup reservoir - that would most certainly cause massive precipitation, and would basically defeat the purpose of adding those chemicals.
I would suggest that you drip the kalk (slaked lime) directly into an area of fast-flowing water, ideally directly into the overflow from the tank, or into the same chamber as the skimmer intake. The baking soda (sodium bicarbonate) and/or baked baking soda (sodium carbonate) can be added anywhere else in the sump (or even into the overflow as well, as the water really moves too fast there for any appreciable increase in pH due to the kalk dripping there (if that is what you're doing...). You could add it to the auto topup as well, as long as you are not adding any kalk or calcium chloride ('turbo calcium') into it.
Can this Carbonate ever be used for the tank's dosing ?
Yes, if you can add it into a calcium reactor. Calcium carbonate will readily re-dissolve at a pH of around 6 - 6.5. It will also become part of the sand bed if you just scrape it off the glass and let it settle out of suspension - or it will be over grown with coralline algae, or act as a natural 'rock' substrate if you leave it to become thick enough (and of course if you have enough light where it has precipitated...)
Hennie