The Inquisition has staff that aren't inquisitors. It would be logical enough for them to hire all those other people that keep the organisation going - secretaries, bureaucrats, legal experts, mechanics, you name it. It's safe to say they'd only select very qualified people, and there would be security measures, but they do need staff and it would make sense for people to be able to apply themselves. People can join the CIA, can they not? Chances are Xeal isn't sufficiently qualified, of course, as very few of us would be, but it ought to be hypothetically possible.
You'd be right about inquisitors, though. One doesn't volunteer to be one of those. You need to be chosen by an existing inquisitor.