Can a community resteem posts from another communities or from another users? This feature would definitely help.

I think I can answer this question for you, but only logically, without knowing the inner workings of communities.

If you use the resteem button at the bottom of a community post, it will resteem the post to your personal blog (meaning the account which is logged in). That works for a community you are part of or one you are just browsing and reading posts.

For a post to be resteemed to the community, I believe, although I'm not sure, that the account owner of the community should resteem the post. The one named "hive-xxxx". Unless there is another option I'm not familiar with. Maybe admins or mods should have this power as well, if they don't?