wordpressのURLカスタマイズ ref: http://qiita.com/halhide/items/9a1ac15164edb92991a9

file0.php

add_rewrite_rule('カスタム投稿名/([^/]+)(/([^/]+))?/?$','index.php?カスタム投稿名=$matches[1]&xxxpage=$matches[3]','top');

file1.php

function add_query( $query_args ) {
  $query_args[] = "xxxpage";
  return $query_args;
}