I guess you already made a /home/knoppix directory in your chrooted filesystem, copied the files in /etc/skel to /home/knoppix, chown'd and chmodded them, then started your xsession as the user knoppix?

From there you can use the kontrol center apps to configure how kde will work.

After you're finished, you'd then rm the files in /etc/skel, copy the modified files from /home/knoppix back to /etc/skel, chown and chmod them back to the way they were, cleanup, and then remaster.