Storyline custom templates NOT custom slides

I am trying to create reusable custom templates (slides) that can be stored in the actual source Storyline files. At this point, the template information I found talks about creating a "custom slide" that has all the features you want in a template and saving that Storyline file and sharing the file with others. To use the template, the Storyline developer copies the "custom slide."

What I would like to create are custom templates that are actually stored within the Storyline source files so that colleagues don't have to use a "stored Storyline file" with custom slides to create a course. Instead, each time someone opens Storyline all custom templates can be found under the Insert/New Slide option. Here is what I want to be able to do: Click Insert/New Slide and from the Insert Slides window have a Custom tab that when selected populates the view with templates I have created. See attachment for example. 

Advantage: as new custom templates are added, people can be instructed to "add a new custom template to their source Storyline files on their C drive" rather than remembering to go grab the latest Storyline file with all the latest custom slides each time they create a course.

Can this be done? I have looked through the Storyline files on my C drive but I can't find from where Storyline pulls the slides  that are displayed in the Insert Slides window.

And, what type of file are the slides in the Insert Slides window?

Any information would be most appreciated! Thank you!