Doctoring a photographic print is one thing. Doctoring a negative in convincing fashion is another. The only approach I can think of is a double exposure, but good luck pulling that off without it being obvious.
Photographic film is remarkable in that way - you can screw with the print, but the thing that's actually captured on the film isn't easy to manipulate once it's been developed.
Doctoring negatives used to be a very common way photographs were doctored. Think about it: you can iterate on the process and have a reproducible product at the end.
I can only imagine the trickery necessary to make this work but the historical record is clear that negatives were definitely altered.
Photographic film is remarkable in that way - you can screw with the print, but the thing that's actually captured on the film isn't easy to manipulate once it's been developed.