Please don't. If I follow one of those groups it's because I have an interest in that particular movie/film/book/sport/whatever. If your posts are no longer relevant I will unfollow you.
Spam about cat litter is just spam.
But it can't be constantly off topic. finding the balance is delicate, and most people do a shitty job.
OP is channeling the average user telling you that if you do this you will be unfollowed.
I'm not sure I agree with OP, it depends on the topic. The HN crowd is not indicative of the general public.
For example, the fashion people would probably be happy to see curated ads.