Install the WP Discourse plugin and enable the SSO Provider option. Doing this will force all users on your forum to login through WordPress.
If your existing Discourse users create accounts on your WordPress site with the same email address they are using on Discourse, they will be able to login to their Discourse account through SSO. Your other option is to export a CSV file of your Discourse users from the Admin/Users page. You can then use a WordPress plugin to import the users into WordPress. There are numerous WordPress plugins that can handle this (search for ‘WordPress import users.’)