I've seen some proprietary lens convertors (wide and tele attachments mainly for digicams and videocams) adopt a snap on system, whereby the lens convertor (say 55mm) screws on to the snap on ring, and the snap on ring snaps into the filter threads of the actual lens, say 58mm. It could be both an inner snap in system (also found in the Nikon Snap on hoods) as well as an outer snap in system - which would be more of a clamp on system found on some types of clamp on hoods.

Anyone know if there are such systems available for use with generic lens attachments?