Safe is a somewhat relative term, IMO you would be defeating the purpose of a bio pallet reactor by adding the kalk to it as the beneficial bacteria might be detrimentaly impacted.
I read the OP as simply wanting to use both biopellets (in a reactor), and use kalk in their top-off water.....not added directly to the biopellet reactor.
And there is no issue running biopellets (simply a food for bacteria to grow on), and kalkwasser in the ATO. Done all the time.