Do you really need a rule to tell you not to take over a plot hook made by a gm? It's not as if they said just make up your own job and do whatever. They made the quest for you to interact with, not as a prompt to write your own story. But again it's up to gm discretion. Still hardly feels deserving to get a reward where the only antagonist is yourself. It's practically a forgone conclusion that you're going to make it out alive, and any chance of you dying or even being severely injured unlikely to impossible.