It is possable, just not very easy.

I tried once, and got to the stage of a working text only knoppix, but then decided it wasn't worth the hassle.

The steps I took were basicly dpkg-repack all the knoppix debs, and installing them on a bare debian install. Then you comress that ect and go on as usual. But it will be very hard to get everything working exactly as well as Knoppix does. Also what is the point?