Is it possible add signature or water mark on photographs without post-processing? Like add the signature/water mark while capturing the photos. Any settings available?
Yes. Although, as mentioned most cameras will require some modification.
With a Physical Mask
The low pass filter on the sensor could be augmented to cast a shadow on the sensor. The easiest way I could imagine doing this would be a metal vapor deposition onto the glass followed by a photolithography and etching process to remove the undesired metal. The resulting shadow mask will add a watermark to each image the camera produces.
An additional benefit of this method is that it can also be used on a film camera.