With the machine being 240 only, you can't convert it to run on 120, however, you could combine two separate 120 circuits together and create one 240 outlet with something like this
Clean Storm Reverse6-50R Power Joiner Step Up Inverter Converts Dual 20 amp 120 volt outlets To 240 volt 3 wire 20amp 6-50R [Reverse6-50R] - Electrical Adapter Temporary Power Boxes - Equipment - Reverse6-50R - Parts & Accessories
However, it is a 240 20 amp outlet, which would likely allow to weld up to about 3/8's, though likely to blow one or both breakers with extended use.
The cheapest, best route, would be to have a 240 50 amp outlet installed close as possible to the main panel, and run an extension cord.
If your main panel has no available circuit breakers, you can install a couple of double circuit breakers to free up space.