I seem to be lucky after all. With my picture, the extension only omits two straight segments, which can easily be fixed manually.

Thank you for your answer. The star is just a testcase. I have a complex shape with many verteces and Bezier curves, which I want to move. I'll see if I can make some sense of the Python code. Is there by any chance another tool suitable for the task? I tried Extrude, but it can't handle Bezier curv...

I created a default star and applied the default motion extension to it. However, the resulting image is missing one edge (see picture). What am I doing wrong?