They could take it from the EcoSport from Brazil. Still, the AWD packaging would likely require a new floor pan in the Fiesta.
Have you ever driven an EcoSport? I don't believe anyone would like that heavy and outdated AWD system in a sport compact built for fun. The system is a great utilitarian system, but would be marginal compared to the built-from-the-ground-up FoRS system. Yes, I understand you mean re-engineering the system for an FiRS, but this brings me to my next point...
If you are already spending money creating an AWD system on the Fiesta/EcoSport platform, it would make much more business sense to then introduce a revamped EcoSport to the North American market with EcoBoost options, AWD and all the plush-lush electronic stuff that consumers like, than invest that development in an even more niche trim slotted above the ST, as currently Ford does not offer the North American market a sub-compact CUV along the likes of the Renegade and HR-V. Could they do both? Ehhh, they could but... a taller ride with tight handling, offroad "capability", and a somewhat larger and ergonomic interior would do well among buyers who live in areas which receive inclement weather (and believe AWD is a save-all), follow an active lifestyle, and/or see a SUV-like vehicle as having higher status than a lowly Fiesta. A Fiesta RS would only cater to those who already have an ST, those who will buy it used 5 years from the release date, and those who just want a smaller, cheaper car with AWD than a FoRS (and I believe, even these people will prefer the EcoSport over an AWD Fiesta, because practicality, common sense, "bigger is better"). I know I'll get flack from this, but the FoRS is a FiRS just bigger, and in Ford's eyes "better", as a FoRS is a halo car with a pedigree and is permitted to be kinda of lunatic, which allows Ford to charge what they do for it. There is only a small amount of "carzy" people who actively look for vehicles that ride harsh, are light i.e. small, and forgo creature comforts in favour of performance and handling. One of them must have slipped the FiST past the bean-counters. We should be thankful that the ST even exists.