The projectile system is hard coded at the moment - you can modify the existing projectiles, but you can't create your own. As an experiment I modified the rpg projectile and turned it into an arrow, with some success. I didn't persist, though, as there's not much point if you can't create your own (the rpg launcher would fire arrows instead of grenades, for example). As far as changing the enemies' reaction to being hit goes, that is possible through scripting, but I'm not the one to help you with that -sorry.
