Can be changed using filter
add_filter('wp_get_nav_menu_items', 'add_utm_to_links', 10, 3);
function add_utm_to_links($items, $menu, $args) {
foreach($items as $item) {
if(!empty($item->url)) {
$item->url .= strchr($url, '?') === false ? '?' : '&';
$item->url .= 'utm=value';
}
}
return $items;
}