Is it possible to cut a threaded hole in wood?
You can drill a hole and force fit a threaded bar into the slightly smaller hole to force a thread, however I would only recommend this option for hard woods, if you are using chip or even ply then I would not risk it. 

I see you ask about 20mm pipe, which would not have threaded inserts which you could use.  The only warning I would have here is that you must have proper bracing on the bars to limit any torsion, i.e. any angular force on applied to the screw thread would easily force the rod out of the hole (at that diameter).. hence there should be three areas of bracing (end, middle, end). 

If you could find 20mm threaded inserts, it would be great also.