I have used these adapters. They work fine for 4 button games (although it only maps X, Triangle, Square, and Circle–you can’t map shoulder buttons to do anything). The PCB itself is programmed to only do 4 buttons, so there is no possible way to mod it to output 6 buttons, you’d have to create a completely new PCB.
So, 4 buttons ok (can’t use shoulder buttons)
6 buttons - impossible.