Of course they can recruit Arbites. I weren't inmplying anything else. But my point was of the first recorded instance in the extended canon, and the Eisenhorn books were first. Strictly speaking, the Inquisitor table-top RPG was first, though.
And Eisenhorn isn't the end-all be-all, because half of the stuff that happens in there is just fluff-faerie-ness and is in no way cannon to the legit GW stuff.
But my point was of the first recorded instance in the extended canon, and the Eisenhorn books were first. Strictly speaking, the Inquisitor table-top RPG was first, though.