@knp
I have not tried the following, but there is this interesting note in Klaus K's description of what's new in Knoppix 7.6.
Why don't you give this a try and tell us if it works for you.

Update: Computers without DVD drive

The flash-knoppix script since Knoppix 7.6.0 supports on-the-fly conversion of a DVD ISO image for direct flashing of a USB pendrive or disk. By using this, the intermediate step of burning a DVD and booting from it, can be skipped. For burning a CD or DVD, only one single .iso file matching your language and version choice, is sufficient. For using this feature, just add the name of the .iso file as commandline parameter to flash-knoppix like this: flash-knoppix KNOPPIX_V7.6.0DVD-2015-11-21-EN.iso