Although Knoppix has a ton of wmmgrs, I want fvwm and THAT it hasn't got.
So... get fvwm from Debian,
Change /etc/X11/Xsession.d/45xsession line #512 to read:
[...] exec fvwm2
Voilá you get fvwm when you boot with desktop=wmaker (or do the equivalent when you boot with 2)
Now for the tough question:

45xsession or /etc/X11/Xsession has something that makes this scheme work. Because you can also: boot with 2 and then say: startx /user/bin/fvwm2 and that gets you also fvwm but... when you try to start mozilla, the latter dies without a trace.... Anybody know what that is?

Thanks!

Hugo