According to this question, it's possible to add the add the exposure time of multiple images. so i can take 20 pictures with a exposure time 1 second and get the same result as a image with 20 seconds exposure time. can i use (in the first case) the same image 20 times? i'm a bit in doubt of this technique, this would make exposure time obsolete.
It is definitely not possible to use a single severely underexposed image and combine it with itself to get a correctly exposed image. The information is simply not there.
The idea of combining multiple exposures is to minimize the noise. If you're using one image the noise will be amplified whereas using many different exposures the noise will, ideally, cancel out itself. Of course in reality this is not the case as there are read noise for example and if the individual exposures are not a lot brighter than the read noise the result will still be poor indeed.
If it was possible to combine an underexposed image with itself it would render the whole concept of exposure irrelevant and the camera industry would look completely different. That is not the case and if it was possible we would surely exploit this possibility.