Is it possible to use existing Canon EF lenses with a Leica body with an M mount?
I've seen various discussions in the Internet on this combination, but usually the other way round (mounting a Leica lens on a Canon SLR body).
Is it possible? Yes. Will you be able to focus it... not really. You won't be able to step it down either. Really... this is probably the worst technically possible combo I can think of.
This site lists the flange distances for most lenses. Any lens system that has a flange distance greater than yours should be able to be adapted without extra glass.
You can see that the Leica M mount is only 27.95mm and should be able to adapt almost any glass given the proper adapter. If that adapter exists, I can't say.