KANekT wrote an extension for adding a topic description - so that's very similiar to what you want.
Here's how his extension manifest looked. Best to probably ask him to code it for you? Maybe offer him a small token of appreciation
[code=xml]
<?xml version="1.0" encoding="utf-8"?>
<!--
/*
* manifest file for topic description
*
* @copyright Copyright (C) KANekT @ http://blog.kanekt.ru
* @license http://www.gnu.org/licenses/gpl.html GPL version 2 or higher
* Donate Web Money Z104136428007 R346491122688
* @package topic description
*/
-->
<extension engine="1.0">
<id>topic_desc</id>
<title>topic description</title>
<version>0.5.0</version>
<description>Topic Description</description>
<author>KANekT</author>
<minversion>1.3.4</minversion>
<maxtestedon>1.3.4</maxtestedon>
<install>
<![CDATA[
$forum_db->add_field('topics', 'topic_desc', 'varchar(255)', true);
$query = array(
'INSERT' => 'conf_name, conf_value',
'INTO' => 'config',
'VALUES' => '\'o_topic_desc\', \'0\''
);
$forum_db->query_build($query);
]]>
</install>
<uninstall>
<![CDATA[
$forum_db->drop_field('topics', 'topic_desc');
$query = array(
'DELETE' => 'config',
'WHERE' => 'conf_name in (\'o_topic_desc\')',
);
$forum_db->query_build($query);
]]>
</uninstall>
<hooks>
<hook id="po_pre_post_contents" priority = "2">
<![CDATA[
if ($fid)
{
if (file_exists($ext_info['path'].'/lang/'.$forum_user['language'].'.php'))
require $ext_info['path'].'/lang/'.$forum_user['language'].'.php';
else
require $ext_info['path'].'/lang/English.php';
?>
<div class="sf-set set<?php echo ++$forum_page['item_count'] ?>">
<div class="sf-box text required longtext">
<label for="fld<?php echo ++$forum_page['fld_count'] ?>"><span><?php echo $lang_topic['Topic Description']; ?></span></label><br />
<span class="fld-input"><input id="fld<?php echo $forum_page['fld_count'] ?>" type="text" name="topic_desc" value="<?php echo empty($_POST['topic_desc']) ? '' : forum_htmlencode($_POST['topic_desc']) ?>" size="80" maxlength="100"/></span>
</div>
</div>
<?php
}
]]>
</hook>
<hook id="po_end_validation, mr_confirm_split_posts_form_submitted"><![CDATA[
if (!empty($_POST['topic_desc']))
$new_desc = utf8_trim($_POST['topic_desc']);
]]></hook>
<hook id="po_pre_add_topic"><![CDATA[
if(isset($new_desc)) $post_info['topic_desc'] = $new_desc;
]]></hook>
<hook id="fn_add_topic_qr_add_topic"><![CDATA[
if (!empty($post_info['topic_desc']))
{
$query['INSERT'] .= ', topic_desc';
$query['VALUES'] .= ', \''.$forum_db->escape($post_info['topic_desc']).'\'';
}
]]></hook>
<hook id="ed_pre_message_box" priority = "2"><![CDATA[
if (file_exists($ext_info['path'].'/lang/'.$forum_user['language'].'.php'))
require $ext_info['path'].'/lang/'.$forum_user['language'].'.php';
else
require $ext_info['path'].'/lang/English.php';
if ($can_edit_subject && (FORUM_PAGE != 'ajax_post_edit')):
$top = forum_htmlencode(isset($_POST['topic_desc']) ? $_POST['topic_desc'] : $cur_post['topic_desc']);
?>
<div class="sf-set set<?php echo ++$forum_page['item_count'] ?>">
<div class="sf-box text required longtext">
<label for="fld<?php echo ++$forum_page['fld_count'] ?>"><span><?php echo $lang_topic['Topic Description']; ?></span></label><br />
<span class="fld-input"><input id="fld<?php echo $forum_page['fld_count'] ?>" type="text" name="topic_desc" value="<?php echo $top ?>" size="80" maxlength="100"/></span>
</div>
</div>
<?php
endif;
]]></hook>
<hook id="ed_qr_update_subject"><![CDATA[
$topic_desc = forum_trim($_POST['topic_desc']);
$query['SET'] .= ', topic_desc=\''.$forum_db->escape($topic_desc).'\'';
]]></hook>
<hook id="vf_qr_get_topics, ed_qr_get_post_info"><![CDATA[
$query['SELECT'] .= ', t.topic_desc';
]]></hook>
<hook id="vf_row_pre_item_subject_merge"><![CDATA[
if (!empty($cur_topic['topic_desc']))
{
if ($forum_config['o_topic_desc'] != '0')
{
$forum_page['item_body']['subject']['topic_desc'] = '<p><small>'.forum_htmlencode($cur_topic['topic_desc']).'</small></p>';
}
}
]]></hook>
<hook id="vf_topic_loop_normal_topic_pre_item_title_merge"><![CDATA[
if (!empty($cur_topic['topic_desc']))
{
if ($forum_config['o_topic_desc'] != '1' )
{
$forum_page['item_title']['link'] = '<a href="'.forum_link($forum_url['topic'], array($cur_topic['id'], sef_friendly($cur_topic['subject']))).'" alt="'.$cur_topic['topic_desc'].'" title="'.$cur_topic['topic_desc'].'">'.forum_htmlencode($cur_topic['subject']).'</a>';
}
}
]]></hook>
<hook id="ape_start"><![CDATA[
define('FORUM_PAGE', 'ajax_post_edit');
]]></hook>
<hook id="aop_features_validation"><![CDATA[
if (!isset($form['topic_desc'])) $form['topic_desc'] = '0';
]]></hook>
<hook id="aop_features_message_fieldset_end"><![CDATA[
if (file_exists($ext_info['path'].'/lang/'.$forum_user['language'].'.php'))
require $ext_info['path'].'/lang/'.$forum_user['language'].'.php';
else
require $ext_info['path'].'/lang/English.php';
// Reset counter
$forum_page['group_count'] = $forum_page['item_count'] = 0;
?>
<div class="content-head">
<h2 class="hn"><span><?php echo $lang_topic['Topic features head'] ?></span></h2>
</div>
<fieldset class="frm-group group<?php echo ++$forum_page['group_count'] ?>">
<legend class="group-legend"><span><?php echo $lang_topic['Topic legend'] ?></span></legend>
<div class="sf-set set<?php echo ++$forum_page['item_count'] ?>">
<div class="mf-box">
<div class="mf-item">
<span class="fld-input"><input type="radio" id="fld<?php echo ++$forum_page['fld_count'] ?>" name="form[topic_desc]" value="0"<?php if ($forum_config['o_topic_desc'] == '0') echo ' checked="checked"' ?> /></span>
<label for="fld<?php echo $forum_page['fld_count'] ?>"><?php echo $lang_topic['Topic enable'] ?></label>
</div>
<div class="mf-item">
<span class="fld-input"><input type="radio" id="fld<?php echo ++$forum_page['fld_count'] ?>" name="form[topic_desc]" value="1"<?php if ($forum_config['o_topic_desc'] == '1') echo ' checked="checked"' ?> /></span>
<label for="fld<?php echo $forum_page['fld_count'] ?>"><?php echo $lang_topic['Topic enable mouse'] ?></label>
</div>
<div class="mf-item">
<span class="fld-input"><input type="radio" id="fld<?php echo ++$forum_page['fld_count'] ?>" name="form[topic_desc]" value="2"<?php if ($forum_config['o_topic_desc'] == '2') echo ' checked="checked"' ?> /></span>
<label for="fld<?php echo $forum_page['fld_count'] ?>"><?php echo $lang_topic['Topic enable and mouse'] ?></label>
</div>
</div>
</div>
</fieldset>
<?php
]]></hook> </hooks>
</extension>
[/code]