Of course you can make your own edits, but you cannot circumvent the game code.
IIRC you cannot make your "Heavy AA" gun fire at its most likely target, heavy bombers, because AA weapons in the game will not fire at those. They will fire at fighter bombers and other low flying "air strike" type aircraft but with a realistic rate of fire, they wont get many shots off anyway. So what you end up with is a rather useless unit that doesn't function realistically.
Just for the fun of it, I tried making a German dual 128mm FlaK and it was rather crappy, getting off a few shots before the FBs turned on it and knocked it out. Not really worth having - but perhaps you'll have better luck
Claus B