No, as a WordPress admin, there isn’t a button that you can press to link a user’s account to Discourse. There is a link on the user’s profile page that the user can visit to link their own account to an existing Discourse account.
When using Discourse as the SSO provider for WordPress, the easiest way for Discourse users who do not yet exist on WordPress to create a WordPress account is for them to create the account by clicking on the Log in with Discourse link on the WordPress login page. This will create a new WordPress user and link their account to Discourse.