If you need a 3.3V supply, in any case your process must provide this option, i.e. a thicker (3.3V capable) gate oxide. If this is possible, you could create your own 3.3V transistors both in schematic, symbol, ..., layout if you know how to take care of, but what about the simulation models? Even if you could get hold of such models from a very, very similar process technology, would you trust them enough for a tape-out? May be for a university project study, but I'd never take the risk for a real company project.