Ok, I found it...

By default there is nos sendfax bundeled with knoppix, and using knoppix as a fax client to Hylafax you need it plus a few other files. This is what I did:

My Hylafax server is a RedHat distro. I copied the following files to my laptop: (as root)

/usr/bin/sendfax
/usr/lib/libfaxutil.so

Created the directory /etc/hylafax and copied:

/etc/hylafax/pagesizes
/etc/hylafax/typerules

...and it simply works. I did not change any defaults on the kdeprintfax command line presented by the system. No other fiddling was needed

I guess the above files have to come from the hylafax server to match exact same versions. Haven't tested other versions yet.

I hope it's usefull to others

Thanks for the attention.