It's done through memory editing the Mount & Blade: Warband server executable, this was pretty much the norm before NW got released. Napoleonic Wars unfortunately isn't as well optimized as Native or Mount & Musket and pushes the limits on the aging engine. There were numerous stress tests prior to Napoleonic Wars being released, and anything above 200 slots was incredibly unstable (latency) to the point where it would crash quite quickly after it got populated above around 218 players. This was presumably the reason why it was released with the 200 slot cap and not 256.
