There certainly are historians who study the occult as a cultural phenomenon that can tell you something about what life or thought was like in another time and place. Check out Alex Owen's 'The Place of Enchantment: British occultism and the culture of the modern,' which is a terrific example of how you can use these subjects to explore the societies that were interested by them. But if you want to study witchcraft and demonology more directly, maybe consider becoming a folklorist?