What happens with these non-mandatory programmes is that a manufacturer can give the programme money to buy their standard equipment vehicle and then the programme will buy the appropriate car from a dealer. The vast majority of vehicles are purchased this way.
This way they can't manipulate the vehicles being tested. The easiest way for your new 4954 model to get excellent NCAP scores is to make the actual 4954's people can buy from a dealer all good enough for excellent NCAP scores, the NCAP will just buy one and test it and it should score well because it's a 4954.
If only there was some way to move cars from one place to another :D
A fellow passenger on a train one day was in that line of work. Customer dispatches him to pick up a car, he gets a train to the nearest station, gets a taxi to the car, picks the car up, drives it to the desired destination, another taxi and another train ride. Pay is reasonable if you like driving and can fill your time on a train (e.g. you like to read or listen to stuff, not great if you're an outdoors person or you need to be at home looking after the kids). Obviously most customers are moving valuable cars they can't get to, classics, high end luxury or sports cars where there might only be two dealers in the country, that sort of thing, but he doesn't care if you want him to drive a mid-market hatchback bought new off the lot yesterday afternoon, pay is the same.
This way they can't manipulate the vehicles being tested. The easiest way for your new 4954 model to get excellent NCAP scores is to make the actual 4954's people can buy from a dealer all good enough for excellent NCAP scores, the NCAP will just buy one and test it and it should score well because it's a 4954.