I disagree with the strong sentiment in most other comments. It's not a total crime to cross-post individual things manually. For example, I've recently de-facto cross-posted this, and it was fine: https://lemmy.ml/post/37434570

That being said, I would absolutely not automate such stuff. I think having to do it manually is a good barrier so that you'd post truly interesting to you stuff, and skip the rest. I wouldn't even participate or encourage software that tries to automate this.