Will skim milk powder go bad when mixed with peanut butter?
I would think that you'd be okay adding non-fat powdered milk to your energy balls. Powdered milk, when mixed with other ingredients, will last as long as the quickest expiring ingredient it is mixed with. http://www.eatbydate.com/dairy/milk/how-long-does-powdered-milk-last-shelf-life/
It doesn't sound as if there is enough liquid in your energy balls to reconstitute the milk. If you are really worried about it you might consider adding a whey protein powder to your energy balls instead of the powdered milk. Although those tend to be more expensive.