Hi Bob,
I want to extend an existing theme and not overwrite it. The suggested way in the description of how to overwrite a theme is to make a copy of the affected file in the special directory and make the desired changes there. Now, when a new version of the theme is officially released, these steps must all be repeated manually.
What I want is to reference the original files (via include etc) and append my enhancements. With this method, a new version is automatically picked up by the theme and only needs to be checked to see if they match the extension.