Is there is a way (without using any external module) to override the core drupal 7 pages access permissions , for example and by default you will not be able to access /admin/structure/taxonomy/tags page without having (Administer vocabularies and terms) permission , but I want the users to be able to access it if they have for example (Edit terms in Tags) permission
You can make it by writing a custom module that implements hook_menu_alter()
.
function mymodule_menu_alter(&$items) {
$items['path/to/change']['access arguments'] = array('permission name');
}