I thought I replied to this weeks ago - I guess not.
I think the closest we could get to this would be to completely overwrite the player with "player A" and then try to re-overwrite the parts that we want to keep from the original player.
This could run into a lot of problems (like crashing the game and/or corrupting your savegame) and I don't see it as an improvement over what is already implemented.
Hmm but surely it's a case of just copying and pasting. If I had visible attributes enabled I could load a player, see his current batting ability, copy it and then load another player and paste in that ability value before applying.
Obviously because I can't see these values as they're hidden I can't do this but if the editor itself could temporarily store the value then it could handle the copy and paste part without revealing the hidden values.
Copy ability would just need to copy the current ability that has been loaded and populate it into a temporary empty box in the editor. I load the second player then I can paste the value from that field into the current ability field and apply. The trick is doing this without me seeing the values.