You could use a motor-generator set up i.e. a 3-phase 415V motor driving a 3-phase 208V generator (with appropriate gearing to compensate for speed/frequency issues).
If you have to ask how o do this, your level of experience is imho not sufficient for such a project. It involves dangerous voltages and power and should only be done by someone with experience or at least under expert supervision. The nearest you come to with a comparatively 'simple' solution is doing as @kellys_eye suggested.