Possessing others is done via the Domination (-2) power, with the Possession (-3) increase. It's also pretty darn evil, infringing on the third and/or fourth Laws of Magic.
If you wanted to stat out a different version you would probably have to specify the circumstances in which you can possess someone else, the methods (if any) for resistance, what aspects of that character to which you gain access during possession, and such. The Ghost Speaker (-1) power might be appropriate, depending on how much of a rapport you want to have with other deceased.
You could probably adapt the workings of the spell "Reading the Dead Eyes" (page 300) to his "temporarily reanimate the dead" power, although again you're smacking up against the Necromancy end of the Laws.