Hmm... I see no point in adding these kind of units to OFP. OFP has one strength: ItÂ's a realistic soldier simulator. The price you have to pay for this is poor graphics and the buggiest game engine ever released commercially. ItÂ's worth it, because of the realism. But a japaneese ninja, with a weapon technology that predates gunpowder seems to me as fitting to OFP as ghosts or goblins.
If these antique assassins shall be able to do anything against anything else than sword wielding barbarians you would have to tweak their abilities to an absurd level. If I want to play unrealistic action games where my character can do everything with a small piece of sharpened metal I can find better games than OFP.
Sorry, but thatÂ's my point of view
Sure, thereÂ's no harm in making the addon, but I still canÂ't see that it would be a very useful one. Rather a bit of fun like the Zombie/Undead-pack or Lega Wars and similar mods.