Recommendation for structuring Help for a system that is stand-alone or an add-on

I am looking for some direction on how to structure a new RH 11 Help Project for a system that can be a stand alone or an add-on to a larger system that already has Help written (RH11)  The original Help was not created as a Merged Help project but could be converted.  Is there another method that would work better?   Is there a way to restrict or expand the Help a user sees based on the system they have?   Thanks in advance for you help.