FtA has y-scrits for healing, that can be easily altered as a integer skill of a soldier.
As for adding a new stat - we have mana, that can do basically anything. Adding more on top of it is a HUGE pain. I did it in GTA and it was a big breaking change.
For instance, can you design a proper and consistent UI for a new stat? Giving how specific it is, you must take in consideration, how any other mod would handle it.