How to add item to an existing selection layer

I have a selection layer of about 150 properties in ArcGIS ArcMap, and I want to add one more building to it (see image, area outlined in red circle). The problem is that I missed this building in particular when I clicked to add all the buildings one-by-one originally.

When I click on it using the Selection Interactive tool to add it to the selection, I can’t seem to make it a part of the existing selection layer called “LargeBuilding selection” (nor can I change its color to green). I’m a relative newbie to ArcGIS, maybe I am missing something. Any help would be much appreciated.