The only way to modify units, of one army, is thru a buff applied to every unit - of that army - as they are created.
While blueprint modifications are easy work - they cannot be applied only to a single army - since, at the time blueprints are loaded, the armies don't exist - AND - blueprints are universal to all players - and unchangeable once the game session actually begins.
For example, the AI uses buffs to give it's units bonuses, much in the way you are suggesting. This work is done at the point of unit creation, and it identifies if that army is an AI army or not. Obviously, you would need to modify this particular logic to provide a path for your buffs to be run on a specific army, human or AI.
