well, I wouldn't say that...the ones with strong African culture most likely practice... I wouldn't say alto due to the fact that most are Muslims and then comes other religion...
I'm not sure if by 'voodoo' you mean sorcery/witchcraft or African Traditional Religions. there are a size able number of adherents to the various indigenous religions in Nigeria. especially among the Yorubas in the Western part of the country.
i don't know if there are any official statistics, considering most forms & applications only give options for Christianity & Islam.