I agree with Tom. If you add a mag. supplement to kalk water, it will most likely cause precipitation problems (calcium and magnesium carbonate).
Perhaps this product of yours uses a quicklime or hydrated lime that is naturally high in magnesium. Personally, I would use regular kalk and supplement the magnesium separately, using the proper mixture of Magnesium chloride and magnesium sulfate and add this mixture to your salt mix water changes each time. Seldom do need to supplement mag. between water changes when you do it this way.