I understand that if I select an object/s and hit / This will isolate the selection. But I would like to know if I can also add objects to the isolated selection?
You can add new objects. Let’s do a simple test 
You can only remove existing selected objects from Local View( Alt + / ). Local View is a quick way to isolate some objects you are working with at the moment. It’s purpose is to be quick. If you want to have more control of hiding and showing objects you are working with you can use collections for that. You have all the control you can think of with collections for that.