i definitely believe that there are a bunch of them hidden throughout the game.
think about it. in konquest mode we have all fought a number of mk characters throughout the journey. The characters all have fighting styles and special moves such as jax, johnny cage, sonya, shang tsung, kung lao, quan chi, etc.
the characters are already made!
why wouldn't they be playable?
it would be a waste for them not to be, it would be stupid.
for the makers of mk to include them as unlockables would take a matter of two seconds, because they are already in the game