It's most likely the baking powder that expires, but I don't know if just adding fresh baking powder will revive it. You need enough acid (milk or cream) to balance the baking powder, or you may end up with a slightly bitter cake.
At only 3 months beyond... I would give it a try, but, there's no guarantee.
1 tsp. should be enough.
"Food is our common ground, a universal experience." - James Beard