Is it possible for someone to become an expert in teaching a subject they have no prior knowledge of?

Yes. You can learn anything on your own. Just do the research. Some subjects are easier to research than others of course. Your prior experience can be related. For example, if you have a background in physical chemistry or physics, teaching yourself metallurgy is much easier because the basics are the same. Otherwise, you also have to teach yourself a lot more to become an expert in the field. It just takes time and effort. It has been done.