Not possible at this time -- it's built as a standalone shortcode or widget. With the plethora of membership/registration plugins out there it's not easy (nigh impossible) to integrate with each (not to mention stay current).
If you already have the info needed to add them to SF (from your registration form), you can do so by:
Calling the submit_salesforce_form() function from your existing registration form (via a hook, or on the success page via a template or shortcode). You may need to populate the $_POST array with data (e.g. if the field names don't match the plugin) or just take the function and modify it to meet your needs.
Or you can just post the data yourself using wp_remote_post('https://www.salesforce.com/servlet/servlet.WebToLead?encoding=UTF-8', $args);
If you're looking for a 'one click' solution, that'd make more sense as an add-on to the member plugin you're using, not to a standalone form generator like this.