Wow... I've never actually heard a setup like this even exists... three phase, sure, we have it here even in households (I go it), but 180 degrees out of phase to get double the voltage... yeah, definitely a first for me.

So, the idea is to use the 240V line or have separate 120V lines? Cuz if it's the latter, just bring neutral over the ground (rewire in a junction box) and use the neutral as ground as well. Sure, not the best scenario, but it should get the job done if most of the stuff you plug in are power tools, which don't have ground anyway.