HDR can't be done in camera on one piece of film as you suggest. HDR selectively chooses pixels from multiple images and recombined them into one composite image. All you are doing with your suggestion is adding more exposure to all grains of the film. Additional exposure is not selective, so it won't achieve the effect you intend.
Essentially what you want to do is compression or expansion so that the image you capture on film can be printed well. This has been done for years using the zone system. You can think of the zone system plus dodging, burning, bleaching, etc. As the analog version of HDR.
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