Hi svenster, I think the least number of steps might be:

Draw a line:


Select just that one line and run Construct > Extrude and click the button in the extrude options for "Set path" and then pick one one of the other edges as the extrude path:


That should generate just one plane. Then select the original object and the new plane and run Edit > Join to glue them together. Then follow that by running Construct > Planar to put in end caps to make it a solid.

So the only thing left over at the end with this way is the line.

- Michael