If a Unit has Shield, or has been granted Shield, and is granted Shield by an additional source, the Shield Value of all granted Shield keywords is summed. Example: Stalwart Poro has Shield. It is chosen as the target of Block, which says "Give a unit [Shield 3] and [Tank] this turn." After Block resolves, Stalwart Poro has Shield 4 this turn.
§ 814.2 Core Rules PDF (officieel)
§ 814 Shield
