I wouldn't.
Milk is perfectly safe but can cause GI upset, so I would only give it in small amounts. I know when I say add water to hard food, I mean if you give 1/4 cup of food then you should add 1/4 cup water. That seems like too much milk to me... Now if you mean just adding a teaspoon or something, that would probably be fine but wouldn't do much for increasing moisture.
Whether you give any milk at all depends on if your dogs can handle it.
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