1.首先数据表 typecho_contents 中新建一个test_url字段，类型可为字符串。

2.后台模板文件write-post.php 表单中插入：

<p><input type="text" name="test_url" value="<?php $post->test_url(); ?>"/></p>

3.在 WidgetContentsPostEdit.php 这里的 writePost 函数里需要接收新字段参数：

public function writePost()
    {
        $contents = $this->request->from('password', 'allowComment',
            'allowPing', 'allowFeed', 'slug', 'tags', 'text', 'visibility','test_url');

4.1 WidgetAbstractContents.php 这里的 insert 函数添加新参数：

/**
     * 插入内容
     *
     * @access public
     * @param array $content 内容数组
     * @return integer
     */
    public function insert(array $content)
    {
        /** 构建插入结构 */
        $insertStruct = array(
            'title'         =>  empty($content['title']) ? NULL : htmlspecialchars($content['title']),
            'created'       =>  empty($content['created']) ? $this->options->gmtTime : $content['created'],
            'modified'      =>  $this->options->gmtTime,
            'text'          =>  empty($content['text']) ? NULL : $content['text'],
            'order'         =>  empty($content['order']) ? 0 : intval($content['order']),
            'authorId'      =>  isset($content['authorId']) ? $content['authorId'] : $this->user->uid,
            'template'      =>  empty($content['template']) ? NULL : $content['template'],
            'type'          =>  empty($content['type']) ? 'post' : $content['type'],
            'status'        =>  empty($content['status']) ? 'publish' : $content['status'],
            'password'      =>  empty($content['password']) ? NULL : $content['password'],
            'commentsNum'   =>  empty($content['commentsNum']) ? 0 : $content['commentsNum'],
            'allowComment'  =>  !empty($content['allowComment']) && 1 == $content['allowComment'] ? 1 : 0,
            'allowPing'     =>  !empty($content['allowPing']) && 1 == $content['allowPing'] ? 1 : 0,
            'allowFeed'     =>  !empty($content['allowFeed']) && 1 == $content['allowFeed'] ? 1 : 0,
            'parent'        =>  empty($content['parent']) ? 0 : intval($content['parent']),
            'test_url'         =>  empty($content['test_url']) ? NULL : $content['test_url']
        );

4.2 这里的update函数里构建更新结构加入新字段:

/**
     * 更新内容
     *
     * @access public
     * @param array $content 内容数组
     * @param Typecho_Db_Query $condition 更新条件
     * @return integer
     */
    public function update(array $content, Typecho_Db_Query $condition)
    {
        /** 首先验证写入权限 */
        if (!$this->isWriteable(clone $condition)) {
            return false;
        }
        /** 构建更新结构 */
        $preUpdateStruct = array(
            'title'         =>  empty($content['title']) ? NULL : htmlspecialchars($content['title']),
            'order'         =>  empty($content['order']) ? 0 : intval($content['order']),
            'text'          =>  empty($content['text']) ? NULL : $content['text'],
            'template'      =>  empty($content['template']) ? NULL : $content['template'],
            'type'          =>  empty($content['type']) ? 'post' : $content['type'],
            'status'        =>  empty($content['status']) ? 'publish' : $content['status'],
            'password'      =>  empty($content['password']) ? NULL : $content['password'],
            'allowComment'  =>  !empty($content['allowComment']) && 1 == $content['allowComment'] ? 1 : 0,
            'allowPing'     =>  !empty($content['allowPing']) && 1 == $content['allowPing'] ? 1 : 0,
            'allowFeed'     =>  !empty($content['allowFeed']) && 1 == $content['allowFeed'] ? 1 : 0,
            'parent'        =>  empty($content['parent']) ? 0 : intval($content['parent']),
            'test_url'         =>  empty($content['test_url']) ? NULL : $content['test_url'],
        );

4.3 select函数里添加查询新字段:

/**
     * 获取查询对象
     *
     * @access public
     * @return Typecho_Db_Query
     */
    public function select()
    {
        return $this->db->select('table.contents.cid', 'table.contents.title', 'table.contents.slug', 'table.contents.created', 'table.contents.authorId',
        'table.contents.modified', 'table.contents.type', 'table.contents.status', 'table.contents.text', 'table.contents.commentsNum', 'table.contents.order',
        'table.contents.template', 'table.contents.password', 'table.contents.allowComment', 'table.contents.allowPing', 'table.contents.allowFeed',
        'table.contents.parent','table.contents.test_url')->from('table.contents');
    }

某些功能简单的字段，例如浏览数，可通过插件实现，参见https://lixianhua.com/typecho_viewsnum_plugin.html

参考：
http://blog.codeinto.com/archives/typecho-two-development-of-the-article-table-to-add-a-new-field.html
http://forum.typecho.org/viewtopic.php?f=6&t=8678
https://www.typechodev.com/index.php/archives/741/
http://www.gaaaa.com/notes/typecho-add-custom-fields.html 这篇文章讲了个最简单的方法，不知道是否符合需求
https://www.typechodev.com/index.php/archives/742/ typecho自定义字段的那些坑儿
https://github.com/benzBrake/ArticleTemplate/blob/master/Plugin.php 这个是利用Plugin增加新字段的例子