WordPress: Add other menu items to existing menu

add-other-menu-items.php

<?php
/**
 * Adds menu items to existing menu.  This supports adding complete menus to existings menus
 * as well.
 *
 * @param array    $sorted_menu_items The menu items, sorted by each menu item's menu order.
 * @param stdClass $args              An object containing wp_nav_menu() arguments.
 */
function add_other_menu_items( $sorted_menu_items, $args ) {
	// Do nothing if there is no menu given.
	if ( empty( $args->menu ) ) {
		return $sorted_menu_items;
	}

	// Get the current menu's term ID.
	if ( $args->menu instanceof WP_Term ) {
		$term_id = $args->menu->term_id;
	} else {
		$term_id = (int) $args->menu;
	}

	// Get the secondary menu.
	$new_menu = (int) $this->get_secondary_menu();
  
	// The existing menu ID to add the items to.
	$existing_menu = 'existing-menu';

	// Perform checks.
	if (
		! empty( $term_id )
		&& $term_id !== $new_menu // Cannot add the same menu twice.
		&& ! empty( $new_menu )
		&& ! empty( $args->menu_id )
		&& $existing_menu === $args->menu_id
	) {
		$new_items = wp_get_nav_menu_items( $new_menu, [
			'update_post_term_cache' => false,
		] );
		
		if ( ! empty( $new_items ) ) {
			_wp_menu_item_classes_by_context( $new_items );

			// Add the items to the current menu.
			array_push( $sorted_menu_items, ...$new_items );
		}
	}

	return $sorted_menu_items;
}
add_filter( 'wp_nav_menu_objects', 'add_other_menu_items', 10, 2 );