OK, my off the top of my head reply got a little ahead of the code, technically. 200.3 says that a premise wiring system "shall not be electrically connected to a supply system unless the latter (the supply system i.e. the secondary side of a transformer) contains,for any ungrounded conductor of the interior system, a corresponding conductor that is grounded." This applies to the situation in question,IMO.
Also, since menegt says he already has an appropriate transformer, all the other suggestions about how he could accomplish the same thing are off the point. He needs 480v 3ph. He doesn't NEED a neutral as the term is being used here. He just needs a grounded conductor, and corner grounding is the simplest way to do it.