>Two or three underexposed (intentionally) images, each having little noise. But adding >up to correctly exposed, and noiseless single image.
I don't believe that's necessarily so.
There are two types of noise, fixed pattern and random. Shot noise is random and best dealt with through blended multiple exposures. But while several underexposed images might improve shot noise, I believe it will make fixed pattern noise such as read noise more apparent.
So in that sense it won't be noiseless, but instead random noise will be improved while fixed pattern noise will be worsened.