Then, you could make the page load from a file inside your extension.
Add your link through 'ca_fn_generate_admin_menu_new_link':
$forum_page['admin_menu']['your_admin_page'] = '<li class="'.((FORUM_PAGE_SECTION == 'your_admin_page') ? 'active' : 'normal').((empty($forum_page['admin_menu'])) ? ' first-item' : '').'"><a href="'.forum_link($forum_url['your_admin_page']).'"><span>Your Page</span></a></li>';
Add one default sublink through 'ca_fn_generate_admin_menu_new_sublink':
if (FORUM_PAGE_SECTION == 'your_admin_page')
$forum_page['admin_submenu']['default'] = '<li class="'.((FORUM_PAGE == 'default-page') ? 'active' : 'normal').((empty($forum_page['admin_submenu'])) ? ' first-item' : '').'"><a href="'.forum_link($forum_url['your_admin_page']).'">Default Page</span></a></li>';
Add the URL scheme for your page through 'co_modify_url_scheme':
$forum_url['your_admin_page'] = $ext_info['url'].'/your_admin_page.php';
Then, on 'your_admin_page.php' inside your extension folder:
if (!defined('FORUM_ROOT'))
define('FORUM_ROOT', '../');
require FORUM_ROOT.'include/common.php';
require FORUM_ROOT.'include/common_admin.php';
if (!$forum_user['is_admmod'])
message($lang_common['No permission']);
require FORUM_ROOT.'lang/'.$forum_user['language'].'/admin_common.php';
//add your language files
$forum_page['crumbs'] = array(
array($forum_config['o_board_title'], forum_link($forum_url['index'])),
array($lang_admin_common['Forum administration'], forum_link($forum_url['admin_index'])),
array('Your Page', forum_link($forum_url['your_admin_page']))
);
define('FORUM_PAGE_SECTION', 'your_admin_page');
define('FORUM_PAGE', 'default-page');
require FORUM_ROOT.'header.php';
ob_start();
?>
//Your content. To get an idea of the markup, check other admin pages
<?php
$tpl_temp = forum_trim(ob_get_contents());
$tpl_main = str_replace('<!-- forum_main -->', $tpl_temp, $tpl_main);
ob_end_clean();
require FORUM_ROOT.'footer.php';
There are probably some errors there because I made it right in this text area, but I guess you get the idea.