you should mount a knoppix iso -o loop and chroot to the image, that is:

Code:
mount -t iso9660 /search/path/to/knoppix.iso -o loop /mnt/iso
then
Code:
chroot /mnt/iso
now you are working in the iso image just as it was your regular install.
that is, you can remove packages, edit files..., i would remove unwanted/non-essentials first, at least enough to give room for your things.
you terminate the session with exit.
for more info man chroot man mount, man apt and man dpkg.
there is more in the customising section. look around there. http://www.knoppix.net/forum/viewforum.php?f=2