AFAIK there isn't a way for Flarum to do this right now (aside from @danielunited's solution, but it doesn't seem to be public). There have been already discussions on this topic: https://discuss.flarum.org/?q=guest%20post. You can browse these for some alternative approaches
It is definitely possible to make an extension for this, but it would require a lot of effort to get it working, due to the way Flarum is built. Someone would likely need to sponsor this feature in order for it to happen.
Edit: You could reach out to Daniel (see above). I don't know if his solution is up to date, but right now it seems that this is as close as we get to guest posts in Flarum
