I don't know If I would add reef plus to the a top off tank. You will not know how much is being added to your tank. top off is really dependent on many external factors such as temperature in your home, how much humidity is in the air and other factors.
Usually additives like that you add is such small quantities and frequencies that you are better off to just add to tank or sump on directed intervals and frequencies.
I only run kalk in my top off system, and even that their is a large difference between the summer and winter months. I usually have to add some calcium to the sump on regular intervals during the winter when my evaporation levels are lower.