How can I include an existing custom sitemaps listing in wp-sitemap.xml?

There are a lot to choose from.

I would start with wp_sitemap_init, whose documentation states

Additional sitemaps should be registered on this hook.

Note that you’ll have to be running WordPress 5.5.0 or up for this hook to be available.

Looking at the code, it appears that this action allows you to register new items in the global $wp_sitemaps object, which is a WP_Sitemaps object.