Customize RSS feed

I'm trying to create a RSS feed with 3 news items and one custom item. This custom item needs to be build manually. I want something like this.

<?xml version="1.0" encoding="UTF-8" ?>
<rss version="2.0">

<channel>
  <title>RSS</title>
  <link>https://www.RSS.com</link>
  <description>RSS Example</description>
  <item>
    <title>News item 1 from views</title>
    <link>https://www.news.com/xml/xml_rss.asp</link>
    <description>News item 1 from views description</description>
  </item>
  <item>
    <title>News item 2 from views</title>
    <link>https://www.news.com/xml/xml_rss.asp</link>
    <description>News item 2 from views description</description>
  </item>
  <item>
    <title>News item 3 from views</title>
    <link>https://www.news.com/xml/xml_rss.asp</link>
    <description>News item 3 from views description</description>
  </item>
  <item>
    <title>Extra RSS item programatically added</title>
    <link>https://www.news.com/xml/xml_rss.asp</link>
    <description>Extra RSS item programatically added description</description>
  </item>
</channel>

</rss>

Creating the feed with views for the 3 news items is no problem. But changing this RSS feed to add one extra item manually is the problem.

I tried changing the views by adding a row...or displaying 4 news items and changing the content of this last row using these hooks hook_views_pre_render, hook_views_post_render and hook_preprocess_views_view_field but with none of them I was able to change the output of the RSS.

I also tried to change the RSS within a twig template. It should be easy to just add an extra item in the views-view-rss.html.twig but adding this to my theme doesn't overwrite the original file at core/modules/views/templates/views-view-rss.html.twig

So now I'm out of options...how can I make