Global sitemap on Multisite

Is there a setting or a hook that I can leverage to create a global sitemap for my WP Multisite installation?

I’ve got a WPMS installation, and I want to treat it like it’s one big site for the purposes of making sitemaps. I’m trying to figure out how I can add the subsites’ sitemap.xml files to the main site’s sitemap.

https://ww.wp.xz.cn/plugins/google-sitemap-generator/