Are questions about hacking games off-topic here? Eg. adding infinite cash etc.
It depends on how you interpret the term "hacking".
Questions about modding a game (i.e. to add to or modify features of an existing game to make it more fun or more challenging) are on-topic.
Questions about modifying a game in such a way that you gain an unfair advantage over other players (i.e. in an online/multiplayer game), then it's off-topic, as it's not game development, it's cheat development.
Note that if you ask specifically about modifying the single player game X, even if it's not off-topic, you might realize that you won't get much answers because that's not really what this community is about, and the amount of users who could give you an answer is low.