For context, I believe you have a popular WordPress site with lots of posts, and have recently created a Discourse forum and added the Discourse WordPress plugin to your website.
How many posts are on your WordPress site? Do older posts receive comments very often, or do people usually just comment on newer posts?
In terms of solutions to the problem, would you consider a solution that required adding custom code to the WordPress site, or are you only interested in ways of doing this that don’t require any custom code?
If posts on the site normally only receive comments for a short period of time after they are published, the easiest possible solution would be to just start publishing the site’s most recent posts to Discourse.
The next easiest thing to do would be to add a plugin to your site that modified the WP Discourse “Start Discussion” link, so that instead of being a link to the Discourse topic, it would be a link that triggered the WP Discourse plugin to publish the post to Discourse. The trickiest part of implementing this would be to figure out how to handle the case of the user who first clicks the “Start Discussion” link - should they be automatically redirected to the topic once it has been published, what will they see while the topic is being published, etc.
Another possible solution that wouldn’t require a custom WordPress plugin, but would require some possibly expensive developer time, would be to get someone to write an import script that would import all of your site’s existing posts, comments, and users to Discourse. That would take care of the issue of empty topics. If this approach was taken, it might be worth considering using your WordPress site as the DiscourseConnect provider for your Discourse site. That would make it easy for the imported users to get access to Discourse.