Finally, after having to spend tens of minutes correcting and converting the first of the real levels from an addon (Tux Strikes Back), I got a working levelset ready to be read by the offical supertux-editor.

NEW: All of the levels have been minimally finished, with some having been altered more than others. Included are both the repacked addon and the levels themselves.
GREAT: The original author Christian Schaefers has now allowed me to license my modifed levelset as CC-BY-SA, version 4.0.

The intro level and the extro stuff will remain unaltered, except for adding my credt for this work.
NOW: The new versions of the levelset are here! I fixed up more stuff as described in replies.