I posted an update, but it seemed to be removed through the forum "upgrade". Anyhow, it turns out the owner of the restaurant found out from the city, that he didn't need to have a patch panel installed after all, so I haven't had to return back to the three phase receptacle job site.
I am fairly confident however, that I could have made an pigtail to use my welding machine off that 3 phase receptacle, though. Maybe I'll have another opportunity where I need to adapt from 3 phase sometime.
'13 Everlast 255EXT
'07 Everlast Super200P