I have never done such a thing but I would expext that it is possible. However you would need insure that the branch circuit was sized sufficently tho handle the additional current. It would not be 125% of 70A and I'm not sure if the calculation would be 125% 77A (230/208*70) or 121A (1.73*70). When you consider the additional power (I sqrd R) loss over time (depending on the amount of use this welder will have) it may be cheaper to buy a boost xfmr.
