I believe the NEC states that the 'ground' and 'neutral' are only supposed to be connected together at the service entrance/main panel. Even at subpanels you are not permitted to connect them together. Typically 220 outlets only have the two hots and the ground and no neutral. There is a good reason for this but I can't remember it off the top of my head. If the 'adapter' is plugged directly into the generator then it won't matter as the generator if wired correctly will have the ground and neutral tied together right near the outlet. I will assume that the generator is properly grounded to the earth.

Ken