wp- multisite XML Sitemaps

I don’t think you’ll find anything like it. Each domain is considered by Google as its own domain. A SitemapXML should only contain pages of the domain it is called from – otherwise it would cause an error and not lead to indexing.

Here is also an answer at Google about this:
https://support.google.com/webmasters/thread/132990940/submitting-a-sitemap-for-a-multi-domain-website-with-the-same-urls?hl=en