Leica CM with external viewfinder

Hello - Im looking at a Leica CM but wondered if anyone has attached an external viewfinder. Does this work, is this possible.
Many thanks,

Leica CM has flash hot shoe, so it is possible. Leica minilux does not.

Off course you can use an external viewfinder. The coincidence of the viewfinder image with the negative may not be 100 % correct, especially with a tele. The distance aside from the optical axis and the height above it is maybe a little bit different from the original camera for which the viewfinder was made. Original Leitz or Leica viewfinders are quite expensive.

What you can do - is to make your own finder (if you will not have zoom, but fixed lens).
Look at this camera - finder is huge, 34mm lens, so close to 40mm. If you want it - it is yours for the price of shipping.

Replying 7 years late, but others may find this useful. Anything inserted into the hot shoe that isn't a recognised flash unit (e.g. a plastic hot shoe cover or an external viewfinder) causes a red "Err" to appear in the built-in viewfinder when 'P' mode is used for aperture control. So, no... an external viewfinder cannot be used if you also want to use 'P' mode. I haven't tested it, but if you're happy to use manual aperture selection, then the "Err" message doesn't appear and the shutter fires when, for example, a plastic cover is in the shoe.