You can add it as a miscellaneous item into the recipe. The water model currently in BeerSmith does not support the use of Ca(OH)2 and so that part of the recipe (water profile and mash pH) will not be accurate for you.
There has already been a message in the suggestions topics to add this into the model, but I am not sure that the developer of the pH model (Mark Riffe) is actively working on it. Papers that Mark has published on his model can be found at http://homebrewingphysics.blogspot.com/