You could build a bridge rectifier from 4 large diodes, and use that to give You DC. However the circutry that allows for start only high frequency probably is not on that machine, so You would have continuous HF or none at all. It would be less grief and probably cheaper to find a Miller 320 or 330 series tig machine [or other older AC/DC machines] in operating condition than to convert this one, they sometimes go really cheap.