This isn't my work; an anonymous user provided this and it's been left forgotten on my hard drive for a long time. I think it's worth pushing it here.
It's a submod, so unpack this into your user/mods directory, launch the game and enable it from the options menu to play it. Adds a couple of mission scripts that allows ninja airfields and outposts to begin spawning on the geoscape before the Pirate/Ninja war is officially declared proper. Compared to a vanilla game, the player would still pursue the typical escalation to encounter ninjas, but will probably see more ninja bases on the geoscape and sooner, as they work towards the war declaration. The submod also allows the player to throttle their progress with escalation, hold short on the war declaration and still have some token ninja base presence.
For those who want more ninja abuse sooner, and for those who only want a ninja-lite experience.
Do note I haven't actually played this and have zero experience with it, only opened it up in a text editor. I don't anticipate the scripts being out of date and broken with current OXCE, but I can't rule it out either. If you're interested, have at.