How can I dynamically add a widget that I can then fetch with a Gtk.Builder instance?

If I want to dynamically add a row to a vbox and populate it... I need a way to see if that row already exists. So the thought train is "Try to fetch the box with builder instance. If it fails, add a row and add the box" If I add the box dynamically, what flags do I have to set or what calls do I have to make in order to be able to fetch it with builder?