Netlify-plugin-sitemap: Manual adjustment on post-processing?

Using GitHub - netlify-labs/netlify-plugin-sitemap: Automatically Generate sitemaps on build seems to work great for a Gatsby-page.
A new released wordpress-blog hosted on a subdomain serves its own sitemap.xml.

How to add a manual entry of the main-sitemap referring the wordpress sitemap?
Perhaps making use of some post-processing scripts that adjust the created sitemap?

mysite.com/sitemap.xml → created using netlify-plugin-sitemap.
blog.mysite.com/sitemap.xml → should be linked from the main sitemap.xml.

Any ideas how to get this done?
Thanks

I haven’t seen a sitemap refer to some other website (in this case, your mysite.com to blog.mysite.com). Is referencing subdomains a standard practice?

That was my first concern as well. Asking bard agrees that this would be ‘standard’ practice.
Anyhow, if the blog would be available within same domain, but not within the gatsby-page, how could the wordpress sitemap be added as a reference to netlify-sitemap?

Well, Bard is wrong then: seo - Sitemaps - one per subdomain or one for the base domain? - Webmasters Stack Exchange. It’s not standard, but a Google-specific feature.

With that being said, the plugin itself doesn’t provide any such functionality. You’d have to add a sitemap yourself OR create another build plugin to add the required entry after the original plugin.

Ok, understood, thanks for the clarification.
Perhaps its best to simply submit the two sitemaps separately to the crawlers.