Option for additional sitemaps?

XML Sitemaps plugin has a feature to add additional pages. Would it be possible (perhaps in some future release) to add additional sitemaps?

I have some folders with static pages under my blog each one of them having a custom made static sitemap. I have a sitemap index in the root folder. Previously it has included both the static maps and the one generated by the plugin.

However, now the sitemap generated is a sitemap index itself. As far as I know, it is not possible to have other sitemap indexes listed in a sitemap index. Easiest way to solve this would be an option to add additional sitemaps from XML Sitemaps plugin but that is not possible (yet?).

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