I'd say Feral ghouls and especially in the number they are in is extremely improbable. Would you truely loose your humanity once you already had it to the point that you would loose the capability to speak. I think yes you could become a vile bastard who just kills people but to become mindless does not seem probable. Freaks would try to find other freaks they would not just become mindless zombies. I'm not stating a definitive fact, just my thoughts on the matter.