It's a matter of configuring XFree86 rather than KDE.
Do a 'find / -name XF86config*' command to find them all on your system.
Also, in the 'serverflags' section of the config make sure that 'DontZoom' is commented out otherwise the control-alt (numberpad) + or - toggle will not work.
If you have XFree86 4.0.1 onwards there is a new graphical configuration client you might want to try - 'xf86cfg'. Otherwise you can use 'Xconfigurator' in Redhat, 'XFdrake' in Mandrake, or 'sax' for Suse. Also I believe there is a utility called 'kvideogen' included in KDE itself.
If you are starting X from the command line with 'startx' you can test new configs by passing them as parms to startx - e.g. :
startx -xf86config /home/user/XF86config.new
Hope this helps