Convert post to article?

Not directly, however, you can split the post out of the thread into its own thread (that is a core xf function) and then convert that thread into an article.

 

You need to set the Forum Moderator Permission"convert thread to article" and once you do that, the option to convert a thread to article will be available in the thread tools drop down as well as thread inline moderation tools

 

I clearly said FORUM Moderator Permission,not AMS permission. Core XenForo Forum Moderator Permissions towards the top of all the user group permissions. It's done that way so you can set that permission via node Permissions to limit it on a node by node basis.