You have a single phase transformer welder. The 415 volt three phase input power will require dropping one hot line to provide 415 single phase. You will have two hot lines to the transformer and an earth ground going to the machine's chassis. You may find the purchase of a bolt on HF unit quite expensive. The negatives will be: no remote fine current/amperage control, no contactor (the electrode will always be hot), a contactor and control device for the contactor activation may be purchased at an additional cost. The biggest negative is arc rectification which turns into heat in the machines transformer. To protect your machine from arc rectification's heat you would be unable to weld at more than 70-90 amps with a 20% duty cycle (my guess from similar machines). At 1 ampere for every 1 thousands thickness that is about 70-90 thousands. 1/16 thickness or so and without the remote amperage control to prevent melting through at the start it is tough. Check out the price of a square wave ac weld output machine versus the cost of the HF with contactor control. Very kindly I would say don't do it.