Yes, you can.
You can use acrylic paint on canvas or pencils, pastels and acrylic paint on paper reproductions. You can add gold or silver foil to areas of the image.
You cannot use anything oil or solvent based, this will damage the product.
Adding content to a reproduction can increase its value and you may call it something like "Enhanced Reproduction" or "Remarque". It isn't a new original, it's still a reproduction so don't try and sell it as an original.