functions.php

<?php
/* Change the WordPress Default Permalinks for Pages
   http://wpforce.com/change-wordpress-page-permalinks/
   ------------------------------------------------- */

add_action( 'init', 'custom_page_rules' );
function custom_page_rules() {
    global $wp_rewrite;
    $wp_rewrite->page_structure = $wp_rewrite->root . 'page/%pagename%.html';
}

?>