As things are now, it is not possible to use the plugin to link existing Discourse comments to a new WordPress post. The plugin works by creating a new Discourse topic when you select ‘Publish to Discourse’ on the WordPress admin page. The linking is done by setting the WordPress post meta-data 'discourse_permalink' to the value of the Discourse topic’s topic_slug.
It looks like it would be possible to add a feature to the plugin to ‘link to existing Discourse topic’. It would have to be supplied with either the topic_slug or or the topic URL from Discourse. That’s not going to happen right away though.