How to add "New > Template Page" as option to existing Design

Hi, I’m helping out a friend that inherited an existing Bootstrap Studio page design. It was apparently started as a Blank Page, not one of the other templates.

He’s been asked to add several new pages that look like the previous pages. Is there a way to make a page and define it as a template so that he can just use the New > Template Page (which offers several to chose from AND automatically adds them to the navigation)

I searched online and only found conversations about making whole Themes for sale, not this specific question.