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Looked into this for you. NO, you cannot reform the Yoruba religion. But YES, you can become feudal if your character has a Stone Hillfort and has implemented the most advanced tribal clan law (most of the time this starts out at level 1 or 2). If you mouse over the law, you will see what else is needed to enact the law. But in summary, one of your segments of society need to be devoted to you, which is the laws below clan levels on the tab. Once you have the most advanced clan law passed, and that hillfort, you should be able to become feudal.
 
DeathLawlie is right, Yoruba is supposed to be already reformed. I forget if this will continue in the map expansion. I know the West African religion has been divided into several religions, but I think Yoruba is still around. If so, it probably is reformed.