1) Simple: Just use magic as is. Another character has pyromancy? Ok, it's not inherent magic, it's bestowed on him by the wizard in question. Use normal rules and use the way his magic works as a justification to let other characters have magic.
For the character himself, same deal: Use the magic rules as they are. If you take refinement, don't describe it as learning to be better, describe it as getting a power back you gave out and it got stronger.
2) Voodoo is just another way to manifest your magic. Standard magic rules apply and should work nicely. Most of the things that make voodoo voodoo are cosmetic when it comes to how to represent it in the game. Instead of a ritual circle you could make a voodoo doll, but the shifts and all that remain the same.
If you want to play up the faith part, just add some faith powers, maybe even make the magic sponsored, not inherent. I've had an NPC like that in one of my games and he worked quite well like that. No real voodoo with him, though.