There is something you can do to check whether a unit is in a transport. That's the same as if they enter a building.
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((Triggering unit) is loaded into (Picked unit)) Equal to True
I don't know how to use it further and it might be complicated. Another possibility is to make an aura with a very small radius (smaller than the collision size for the building). This way (not sure) only units who enter then building will get the aura.