processing-to-on-hold-email.php

function add_another_email_action( $array ) {

  $array[]='woocommerce_order_status_processing_to_on-hold';

  return $array;
}
add_filter( 'woocommerce_email_actions', 'add_another_email_action' );


function hook_another_email_on_hold( $email_class ) {

  add_action( 'woocommerce_order_status_processing_to_on-hold_notification', array( $email_class->emails['WC_Email_Customer_On_Hold_Order'], 'trigger' ) );

}
add_action( 'woocommerce_email', 'hook_another_email_on_hold' );