There's no real way to do it, but if you really wanted a non-coding method, you could give every unit in the game its own armour type, so they'd all have just one type of armour, allowing each one to still have the strengths and weaknesses of their original armour, with an added value for magic resistance. Then you could use
GAE's techtree/faction translation feature to make this unique armour type have the same name as before.
For example, "unit_a" has the armour type "unit_a", but in the techtree XML, you state that in-game, it will appear as "metal". This way "unit_b" can have the unique armour type "unit_b", yet it could also appear as "metal" in-game. Since it's actually a different value, you can assign different damage multipliers (including one when being hit by magic) in the
techtree XML. In-game, the text (in the unit's info window) will still read "metal", even though the actual type is "unit_b". If that sounds confusing, just remember that a translation doesn't affect the data at all, it simply changes the text string that's shown in game.
