I've got a method for that. I was able to add evasion and critical strike to Agility by making a hidden passive ability.
For armor on Strength:
1) Disable the armor from Agility. Do this through Gameplay Constants.
2) Make an ability based off the "item bonus armor" ability.
3) Give this new "Armor Bonus" ability as many levels for each point of armor you want it to give. You'll have to make your own formula for it based off the maximum strength any hero can get.
For example:
If Maximum Strength = 200, and 5 Strength = 1 Armor, then make the ability have 40 levels with +1 Armor for each level.
4) Add this ability to your heroes.
5) Make a trigger! When a hero gains a level, check its strength and increase the level of "Armor Bonus" accordingly.
6) For strength from items, make another trigger! Use the events "Unit acquires item," "Unit loses an item," and "Unit sells an item" for the trigger, and do the same for step 5 to adjust the "Armor Bonus" ability.
*special extra* Sort through the Game Interface menu and change the info for both Strength and Agility to be more accurate to the new stats.