Sitemaps belong to a domain. Not sure if there is any special code that lets you extend the main site’s sitemap, but you can always use the sitemao directive in robots.txt “file” of your domain, either by editting it “manually” (probably one of the SEO plugin can help with doing it without breaking anything) or by adding a plugin that will hook on the 'robots_txt' filter and add it progrematically (which is how the main sitemap is added to it)
