That technique works pretty well to change from N phases to M phases, where N is greater than 1.
You simply use a wound rotor motor, with the stator wound for N phases, and the rotor wound for M phases, and lock the rotor in position.
In one of my worksites, we used a wound rotor induction machine, which looks exactly like the transformer that you describe, with a three phase primary and a three phase secondary. The rotor was anchored to a gear box that let us rotate it, but would hold it in whatever position it was placed. By changing the rotor position we would change the phase of the voltage induced in the rotor coils. The rotor coils were connected 'buck/boost' to the feed lines, giving us a nice variable voltage three phase output.
-Jon
