[magick-users] Convert TIFF 6 -> Tiff 4 + + special conditions

[fk_bln at arcor.de]

I would like to do the following:

Convert an image file (TIFF 6.0) to TIFF G4 (black/white right?) and the black pixels shall be black after converting the white ones shall be still white. But all non black pixels (arbitrary color except white) shall turn to white!

My questions are:
-	Is this possible with Image Magick?
-	Could anybody provide the options I have to use to achieve this goal?

Many thanks in advance

Frank