If you had a photocopy of the stamped signatures you could position the paper over the photograph and simply trace over the signatures so an indentation was left on the photograph. Then using a felt or wider tip pen you overwrite on the indentations making it look signed. You most likely would need to go over some areas several times to make it look correct?
That’s certainly possible. You’d have to have patience and skill to make it turn out that good though. Obviously it isn’t great, but it’s better than I would expect doing it that way.