Yes, the WRC level/class rally Fiestas all start as
base Euro/rest of world 2 door (and rarely 4 door for some of the R2 builds) Fiestas, this includes Ken's wife's R2 which she still campaigns in ARA events occasionally.
The FIA may even allow them to start with a base 'body in white' SHELL of the unibody to save them all of the work of stripping it down, but probably not since they must have a factory serial number. since they are street licensed and insured in the country of origin/build to be able to drive on public roads during the 'transits' between the at full speed, competitive stages.
They then add in all of the FIA approved/homologated 'kits'/parts/engine/drivetrain/bodywork to build them to whatever WRC/European/National Championship class there are going to be competing in.
They do NOT ever start out with an actual ST.
But Team O'Neil did campaign, and WIN Rally America Championships in a fully prepped FiST in the Open 2WD class (some engine tuning allowed, full Reiger suspension, Sadev sequential gearbox, M-Sport RS2000 rear wing, etc.) with Andrew Comrie-Picard driving.
This is the car pictured with that wild livery scheme on Mountune's site in the Fiesta ST header section, and also on that popular winter video floating around on you tube comparing this car to a base FiST on winter tires.
Ken did have a/some
rallycross car(s) which sported FiST
badges, but these were NOT built from STs, as wheel to wheel rallycross cars are even MORE radically different from street cars than even the top level, WRC Rally 1 cars (inline, 'north-south' engine layout, different AWD sequential drivetrains, and 650+ HP output, huge snail, 2 liter powerplants, as well as no lights, horn, or turn signals, since they are never driven on the street like WRC cars are.