How to make black peppercorns extremely fine?
Better grinding and sieving

If you think about it - spice powders are essentially flour. Pop it in a good blender. Blend, let it settle and sieve to required consistency. Repeat with oversized particles.

In an industrial setting this would likely mean a large mill and vibrating sieves, but you can, very carefully get the same results on a smaller scale. I believe some folks also use coffee mills for this, though obviously not one you have or intend to use for coffee.

For small quantities, you could probably use a spice mill or mill attachment on a blender. Just take your time. 

Its also worth remembering a pepper mill cracks not grinds pepper.