The most likely way, and probably the way I will be implementing it, is having full or near-full Donkey-Brained players have the option to try and infect other Humans, with choices that fight the desire to do so becoming less available the higher DB level you are. Nine times out of ten, this would probably lead to a specific ending associated with the location and character, but ideally (and unlikely to be put into the game) this could lead to a branching story path where Amanda leads a herd of Donks throughout the facility, reigning havoc on any Humans in her way. Furthermore, an ideal version of that is being able to freely travel across the facility in gameplay, hunting down Humans in control of Amanda, a semi-failure of this could very well be a unique ending where Amanda could lack the Infection levels to be recognised by the other Donks and be dulled into the masses.
If it's obvious by my brainstorming, nothing is concrete for the story (and never has been :p) I just write in the moment and see where it leads, then go back to it later. So, TL;DR: I could very much add a sequence of events where Amanda goes Donko-mode and infects others, but we'll have to wait and see ^^"