Now I tried to create a new coordinate system that is a copy of an existing one - but with units cm instead - to see if this "tricks" FME to think it is already in cm - even if it is in meter.
The issue is that the opening software (Navisworks) seems to try to convert the geometry but not the material/texture/raster-coordinates. Anyhow it might be usefull to be able to write to m instead of cm.