I agree with zombie84. Film grain is a natural byproduct of photography and varies from shot to shot, depending on the film and lighting conditions. To simply add a layer of grain would betray its artificial source. And, surely the issue is not of the lack of grain itself but rather the detail that was removed along with the grain? Adding grain back into the picture will not restore any of that lost detail, no more than adding hiss to NR'd audio would enhance its texture or clarity.