We know that adding <!–nextpage–> allows you to paginate the content of the post/page in WordPress.

Also if you want to append the content you can use the_content filter.

This filter though works but adds that content to all the pages instead of creating a new page for that content.

For example: I have a paginated post with 3 pages, separated <!–nextpage–>. I want to add an additional, fourth page, the_content will not allow that, so in that case, you can use the following code to add a new page to your paginated post.

[php]
add_filter(‘posts_results’, ‘change_the_posts_content’);
function change_the_posts_content($posts) {
if (is_array($posts)) {
foreach ($posts as $post) {
if($post->post_type==’post’){
$post->post_content .= ‘<img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" data-wp-more="nextpage" class="wp-more-tag mce-wp-nextpage" alt="" title="Page break" data-mce-resize="false" data-mce-placeholder="1" />test test test’;
}
}
}
return $posts;
}
[/php]

*You will need to change the post_type=”post” to post_type=”page” in order to get it to work with the pages.