Adding directly to your sump wil work as long as it's just a little bit in relation to the water volume (maybe 1 tsp at a time per 100 gallons would be safe IMHO.) Kalk will raise the Ph pretty dramatically - especially in the localizewd area that it is added. Also, one of the advantages of pre-mixing is that any heavy metal impurities will have a chance to settle and you can discard the sludge at the bottom of the resevoir periodically.