Looking for all the places I would have to change the base VX Ace codes for Parameters to make them ADDITIVE and not MULTIPLICATIVE.
Parameters do add. I'm sure you've already taken a look at the param_plus method in Game_Actor. And if you've checked the actual parameters array of the items, you'll see they comprise only the flat stat values for the eight stats you see in the databse.
What you're thinking of are Features. Whether those values stack multiplicatively or additively depend on the actual feature itself. In the case of parameter change features, they're positive percentages, so of course they multiply. Otherwise when wearing something that doubles the stat (200%) and something that halves it (50%) you'd get 250% stat instead of 100%.
What you REALLY want is not just to make them stack additively, but change how those percentage numbers are seen by the engine. I suggest you take a look at the features_ methods in Game_BattlerBase, particularly features_pi and param_rate. Good luck.
Edited by Traverse, 10 December 2016 - 12:04 PM.