If a 2015 Prius Plug-in has PCS, can the pedestrian detection system be ”added” easily or are they very different systems?
Yes very easy. Trade it in for an '18 model. Honestly, modifications to a computer system like that usually are so complicated that changes like that are practically impossible. And once modified it might not be reliable. NOT a good idea.
Well you'll need a camera, the associated ECU and the software to talk to one another. There's also the possibility that the radar unit itself is different between the two systems. (I noticed that the radar unit on the Prius 4 is much closer to the emblem (and therefore the front of the car) than on my Prius 3 which is set further back.