[Plugin: WordPress SEO by Yoast] include non-wordpress pages in sitemap

This more like feature request.
Since blog is installed in root of domain, would be nice, if in plugin would be place where to add non-WordPress sites.
Otherwise now they all are dropped out, because of sitemap_index.xml which (at least) I used for such pages.

To simplify: just one input text field, where to add new sitemap of such links.
Example: https://rolandinsh.lv/sitemap_projects.xml

https://www.ads-software.com/extend/plugins/wordpress-seo/