While one cannot drop a .gha on the Explicit History canvas on osx (yet), one can still install some third-party add-ons by adding the .gha to the Libraries folder. Just go to File > Special Folders > Components Folder and drop the gha in there. Then, restart Rhino / Explicit History.
I tested with Meshtools.gha and all of the functionality is there, but no icons appear. YMMV with other third-party add-ons, so please, let us know what is working and what is not.