How are you doing this?
Not yet, but it wouldn’t be hard to add an extension to the WP Discourse plugin to do this. How many users are on your site? Are they all being added to Discourse?
The easiest way to get your WordPress users onto your Discourse forum would be to use WordPress as the SSO provider for Discourse, but unless you have a reason for users to be logging into WordPress, migrating the users to Discourse would probably be a better approach.
You could also just send invites to your WordPress users through the Discourse admin section. You would risk losing some users by doing it this way.