Well, to burn knoppix you can use cdrw's just like for any ordinary iso, no problems there

however, writing to cdrw is a whole different story, as everything needed to burn cd's is stored on the cd itself! A possible workaround would be to load (a part of) the image into RAM, and burn from there, or to use a cdrom/cdwriter combo and use a second cdrw (but that wouldn't be a real challenge )