not sure about ncap,
but I think the nhtsa requires have certain requirements for cars that are imported to the USA.
and you are only required to undergo testing if you have sold >10k cars.
prime example was the Porsche CGT, it has to be refitted with dual airbags to be sold in the USA. However, sales did not exceed 10k, so it did not undergo testing.
The audi RS4, had to abandon it's recaro seat also.
*info above are only what I remember from my friend's discussions, which may or may not be true