I developed a bot for this purpose, it only crossposts links to external websites like news, it's being used in a couple of communities that need a little push with content.
It only crosspost 1 post every hour to each community to avoid spamming and it can handle multiple subreddits at the same time.
That's how I think it should be done, so far, users engage with the posts made by the bot in an organic way.
If you want to check it out: https://github.com/daniel-lxs/BotIt
I'm open to suggestions :)