Creating custom buttons to display in button tab

There's no way to add your custom button to the SL ribbon. 

Here's what I do: I add an extra scene called "Storage" to the course. I put a copy of any useful objects in that scene. This might include custom buttons, specially formatted shapes, etc. 

That makes it easy to copy any item and paste it on whatever slide is needed. 

When the project is done, the extra scene can be deleted. Or keep it for possible use during revisions. Just be sure to remove it from the built-in Menu if you're displaying that in the course.