This could probably be done by setting up a custom drag and drop event for ZIP files dropped on the ExtensionsManager window.

Or even better, a scripted file dialog class can be registered (similarly to this). If the class recognizes the file as an extension package then it can offer to install it (similarly to this).

Zip file handling is already hijacked in qSlicerDataDialog. This special case handling may need to be replaced by a standard FileDialog mechanism, otherwise it could be difficult to ensure all plugins have a chance to analyze the drag-and-dropped files.

It would be also nice to handle drag-and-drop of .py files (load as module if it has a correct module class name, also add it to additional module pahts; otherwise run it as a script).