What do you mean, do you want a passive ability that gives armor to the unit? Then you could just modify the item ability "armor bonus" and make it a unit ability and add tooltips etc...
Go for: Object editor -> the unit you want to increase the armor -> Combat -> Defense upgrade bonus
change the data 'defense upgrade bonus' to whatever you like, every upgrade which does 'Apply defense upgrade bonus' will increase the armor of your unit by the amount you've choosen for every unit individual.
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