If the recipe called for 1 tsp. of baking powder and 1/2 tsp. baking soda, I would have just doubled up on the baking powder or added 1/2 tsp more, if it was a cake and I was really desperate, haha! But for cookies, I wouldn't even try.
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How much baking powder do I have to substitute for baking soda?
Baking soda is four times as strong as baking powder so if your recipe calls for 1 teaspoon of baking soda, you would need four teaspoons of baking powder to produce the same amount of lift. Unfortunately, though, its not that simple.
Baking powder is made of baking soda and exactly the right amount of acid to react with the soda (it also includes corn starch to keep the ingredients from prematurely reacting in the privacy of their container). So if your recipe already has acidic ingredients that were going to neutralize the baking soda called for, you are adding other ingredients that may not sit well with them.
Substituting for a lack of baking powder is very easy: 1/4 teaspoon of baking soda plus 1/2 teaspoon cream of tartar (ignoring the cornstarch) for each teaspoon of baking powder required.
Weve looked in dozens of books to be sure, but no one provides information for the reverse procedure substituting baking powder when you dont have soda on hand. To do so, you would have to consider the acidic ingredients in the recipe, and perhaps reengineer the recipe to replace them with more neutral ingredients (using whole milk instead of buttermilk, perhaps). But at that point, you would see, it would be easier and probably a lot more successful to pick up a box of baking soda.