> Yes, it would be really easy. You "just" have to be able to buy thousands of official controllers and solder a microcontroller on them. And then resell them. Without getting sued.
That's exactly what people did back in the day, First-sale doctrine allows for this (Heck some modders I knew back in the day simply brought them in bulk at wholesale). Modders either brought controllers retail, modded them and put a mark up on them, or people would send in their own controller to be modded.
But those selling wired K&M convertors just decided they would pass though the challenge/response commands to a offical wired controller the user would have to purchase themselves (saves the manufacturer the need to purchase them).
> Or teach people to solder the microcontroller themselves.
There were plenty of sites with open source rapid fire microcontroller firmware and install guides for those who didn't want to pay others to mod their controller for them. It was simply VCC, GND, each of the trigger inputs, a connection to the sync button, and a connection to one of the LEDs to give feedback to the user which mode the controller was in. Heck it was on the skill level needed to mod a PS1 and those services were ten a penny. Heck it got to the point where drop in PCBs where being created where castellations on the PCB were in just the right spot to solder.