@kate11, that's a different thing (Yeah, I used to play KalOnline). The way I'm thinking is that, when you visit a potion seller, you talk to it, and then choose the green option to open up a potion store. Basically, I'm trying to figure a way to add that option to a non-potion selling NPC. Because, the NPC would call up the potion window and you choose what you want and the then the server and client exchanges information. If it is done this way (I don't see any other way), then the only thing that could stop this is server-sided NPCs. But, I did succeed in changing their descriptions. My only problem now is locating where those options are stored for each NPC.
@sixshooter, in theory, redoing quests would be the same idea as adding a potion shop option to a useless NPC. but, the problem I see with that is that the server tracks which quests you've completed. You can tell just from the Quest Log. Editting the rewards might be possible by editting the right data. However, changing the level requirements of a quest wouldn't work in my opinion. All the level requirements I've run into the .csv files are server-sided (dungeons, for example). But, then again, I remember you could change the level requirements of armors while back...
Well, as a final thought, the only way for any of this to be do-able is whether or not the client actually sends a request for an item and the server reacts and responds. I believe this was the case in some other games (but, most of them were banned soon though). Heh, I'm too lazy to learn computer programming and such just for a game. Maybe someone else could help out with that.