Where are the default matcap files located? Is there a directory or path that you need to follow to access the folder with all of Blender's default matcaps in? If so then is it possible throw in new matcaps?

I'm using Windows 10.
You can't add matcaps in blender 2.79 or lower, as in the release notes of Blender 2.66
(March 6th, 2013) it read:

Currently a set of 24 (GPL licensed) Matcaps have been compiled into Blender.
[...]
Loading own matcap image files will be added later. That needs design and code work to get it stable and memory-friendly.


6 June 2018: This option has been added to the development version of Blender 2.8 as shown in this youtube preview by the Blender Developers