You can add to that menu via Settings / File Types, expand System File Types and edit the context menu for (NONE) Drive, near the top of the section.
Since the menu for drives comes from Windows rather than Opus, any commands you add should use the "Run an application (supported in Opus and Explorer)" type.
If you need to run an Opus command using that type, you still can using dopusrt.exe. For example, this will run the Help ABOUT command:
"C:\Program Files\GPSoftware\Directory Opus\dopusrt.exe" /acmd Help ABOUT
(Edit: As per below, you can use %1 to insert the path to the drive being right-clicked into the command.)