Well, I reasoned that there HAD to be a way of editing tdbsam from the command line and took it from there.
This was reported as Bug #16575 in kdebase (Debian) - "Cannot create samba user with kcontrol samba module" in 2005 -
https://launchpad.net/debian/+source/kdebase/+bug/16575 - !
From the reply on
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=293960 it seems to be a problem in KDE 3.5.1, "the Apply button is not enabled after adding a UNIX user."
Strange that such an important bug affecting a mixed OS environment has not been seemingly generated so little interest or been reported more.
Maybe KDE users just use simple file sharing all the time (no good for a serious environment).
I'm taking the matter further.