Мой ответ выше для добавления сообщений.Ответ ниже - добавить страницу (она добавляет страницу активации темы):
if (isset($_GET['activated']) && is_admin()){
$new_page_title = 'This is the page title';
$new_page_content = 'This is the page content';
$new_page_template = ''; //ex. template-custom.php. Leave blank if you don't want a custom page template.
//don't change the code bellow, unless you know what you're doing
$page_check = get_page_by_title($new_page_title);
$new_page = array(
'post_type' => 'page',
'post_title' => $new_page_title,
'post_content' => $new_page_content,
'post_status' => 'publish',
'post_author' => 1,
);
if(!isset($page_check->ID)){
$new_page_id = wp_insert_post($new_page);
if(!empty($new_page_template)){
update_post_meta($new_page_id, '_wp_page_template', $new_page_template);
}
}
}