None of the built in units will correct the original signal ... just the hamonized voices. Antares makes a unit that does just that, but it's not cheap, not perfect and not easy to operate in a live setting.
My advice? Use lots of effects on the voice and "hide" in the wash !
Lots of people are making money with lousy voices everyday:
Cher(yuk!), Bob Dylan, Britney Spears, Kathy Lee Gifford (double yuk!!) lots more ....