Adjusting weapon ability modifer

I am trying to homebrew a spear for my campaign that utilizes Charisma for both to hit and to damage instead of strength. What modifier would I need to select in order to make this happen? Any help is greatly appreciated

Why do so many people want to do this?

There isn't a modifier for that currently.

Thanks! It allows more freedom within your game. I am trying to give my sorcerer something other than the standard staff or wand.  

Their cantrip damage is probably as high as a weapon's anyway.