Thank you both for your constructive thoughts.
My solution is a little different. I have created the 7 subdirectories of Scenery for the the continents, and some of these already contain non-Orbx sceneries. Orbx will not allow me to create pointers to these libraries if they are not empty.
I have therefore created an additional dummy 'continent' and temporarily moved each of the existing continents' contents to it while I create the Orbx library pointer to it, then moved the dummy's contents back. Using the File Explorer>Move to icon, this presented little workload.
There remains a little missing facility - I am unable to view the contents of the Orbx scenery libraries from the libraries listed in the Libraries display. And I am still at a loss as to what Scan for Libraries might be meant to do.