You want to integrate WordPress comments into Disqus comments system.
1. In your WordPress Dashboard, go to Plugins > Add New. Search for Keyword = Disqus. Ensure the Description is by Disqus and you can click Install Now to install the Disqus Plugin.
2. Once installed, you need activate the Disqus plugin. Then, go to Plugins > Installed Plugins. Look for Disqus Comment System and click the Settings. You will need to register an ID with Disqus. After the registration is completed, you can start to import your WordPress comments into Disqus!
3. Go Plugins > Installed Plugins > Disqus Comment System > Settings. Scroll down till you see below screenshot and click Export Comments.
4. The export will processing all the comments in your WordPress to Disqus. You can see the status at the link provided. Normally, this will take almost 24 hours to fully migrated to your Disqus (depends on your comments size!).