There's no such thing as a "passive ability" in StarCraft II. They are behaviors now.
Fortunately, behaviors can easily be given a limited duration, and you can create abilities that add behaviors. The effect/behavior approach should have worked. Can you tell me what exactly went wrong? Did you remember to add a button to the Command card? Did you remember to give the behavior a limited duration?
If the button appears and can be clicked, but nothing happens, then I will have to give it some more thought. However, telling me it just doesn't work is too vague.