Knoppix already has a mechanism to do basically what you would like to do. I'm actually a big fan of customizing Knoppix without remastering and with AuFS and the persistent disk image you can do a lot of changes with Knoppix without a full remaster. I covered these techniques a bit in the first Knoppix Hacks but that was before AuFS/UnionFS. In the second edition though I really expanded because you could just do so much without having to open up that KNOPPIX file and tweak it.

At a basic level you can achieve what you want by tarring up the files that you want to add, including any packages, etc. into a tarball that you place in the KNOPPIX/ directory on the disc (probably DVD since the CD is pretty full). Then just create a knoppix.sh file in that directory that extracts the tarball and runs any other programs (like installing any .debs, etc.) that you want. The knoppix.sh script will be run at boot.

Or you could just make the changes you want to make and then add them as part of a persistent disk image, and then add the persistent disk image to the DVD itself if you don't want to carry it around on a USB disk.

Just a few ideas. I've actually been able to do a lot with this "remaster without remastering" method including setting up a custom Knoppix CD as a local webcam server.