注册自定义帖子类型时，可以将分类作为参数添加到register_post_type函数中

add_action('init', 'register_CPT');
function register_CPT() {
  register_post_type('CPT',array(
    'labels' => $labels,
    'public' => true,
    'publicly_queryable' => true,
    'show_ui' => true,
    'query_var' => true,
    'rewrite' => true,
    'capability_type' => 'post',
    'hierarchical' => true,
    'menu_position' => 5,
    'taxonomies' => array( 'topics', 'category'),
    'can_export' => true
  ));
}

但如果分类和自定义帖子类型都存在，则可以通过register_taxonomy_for_object_type函数将每个分类添加到CPT中

add_action( 'init', 'add_taxonomy_to_CPT' );
function add_taxonomy_to_CPT() {
  register_taxonomy_for_object_type( 'topics', 'CPT' );
  register_taxonomy_for_object_type( 'category', 'CPT' );
}