If you are planning to use WordPress as the SSO provider for Discourse, and also want to have the Drupal posts on your WordPress site, there is a Drupal to WordPress plugin. The documentation for it’s paid version says that it will import users and authors. If you took that approach you would need to publish your migrated posts from WordPress to Discourse through the WP Discourse plugin. Publishing 1500 posts manually sounds like a lot of work. It might be possible to write a script to do it automatically.