TC-14E II with Leica-R
The Leica-R lenses converted to Nikon mount can use the Leica-R extenders only when irreversible surgery is done to the extender.
The best alternative are the modern Nikon teleconverters.
From October 28th 2013, the Leitax mounts for Nikon are compatible with the excellent  TC-14E II, TC-17E II and TC-20E II/III

The front elements of these extenders poke out, so you can use them only on long focal Leica-R lenses, that have the rear elements deep inside the lens barrel.