function alt_emails_for_attendees( $update_data, $registration ) {

    if ( ! $registration->is_primary_registrant() ) {

        $email_parts = explode( '@', $update_data['user_email'] );

        // Convert email@domain.com to email+firstname@domain.com

        $update_data['user_email'] = $email_parts[0] . '+' . strtolower( $update_data['first_name'] ) . '@' . $email_parts[1];

    }

    return $update_data;

}

add_filter( 'wpf_event_espresso_customer_data', 'alt_emails_for_attendees', 10, 2 );