Crossposts already exist, there's a button for it. For example, I just crossposted this post so now it shows up with a little cross-posted to: World News and my cross-post on World News also shows a little cross-posted to: environment@beehaw.org
I would love to see this as well. It's definitely a challenging topic as crossposts can happen across instances, but I think it would greatly improve the discoverability of content and new communities.
Depending on exactly what you have in mind as a cross-post feature, there is this button on posts to cross-post: (also I see other folks have answered this, but seems like the replies didn't arrive over here on my derpy instance)