This can now be done with the WP Discourse plugin. Any WordPress post that is not yet linked to a Discourse topic can be associated with a topic by selecting the ‘Link to Existing Topic’ option from the post’s Discourse meta box.
Once a post has been linked to Discourse, it will behave in the same way as if you had published the post from WordPress to Discourse. Posts can also be unlinked from Discourse and associated with different topics.