Disclaimer: what's written below applies to v83.
The skill type (eg. targeted or straight-line, if we're talking about projectile skills) is handled (and hardcoded, courtesy of Nexon) clientside.
You technically can change the way they work, but you need to modify the client in different places.
Turning a targeted skill, such as Holy Arrow, into a "linear aoe" (I'm sorry, I don't really know how to call them) like Fire Demon, for example, requires you to:
a) include Holy Arrow among the skills that throw a linear projectile, and remove it from the list of targeted skills (there's one hardcoded skill ID group for each class, if I recall correctly);
b) add Holy Arrow among the skills that need linear projectile display (otherwise it would fire an invisible shot);
c) add Holy Arrow among the skills that need to have their projectile path dictate which mobs are going to be hit (otherwise it would behave like Dragon Roar, picking mobs "randomly" around you).
Ofcourse, apart from the client edits, you'll need to add the proper wz-side values for mobCount, lt and rb (and maybe one or two more that I can't remember at the moment).
Basically, you'd have to follow the route of another skill that works in that way, both wzside and clientside.
The main issue is that there's no "free room" in the client around those checks, so you would have to codecave it in.