I think it's very likely Waltz is the one who added him, but I think it's unlikely to have been intentional. I think he probably fat-fingered it.

Based on how he tried to smear the journalist, and implied that the contact was injected, you shouldn’t extend maturity to him.

Many media outlets reported Trump was mad that any of his staff had reporter contacts in the first place. If your assertion is true that would be the quick end to Waltz's short career.