To import sitemap from XML file (beta):
*For security reasons import from XML is limited to 500 pages.
Importing sitemap XML file feature is only available to Pro, Team and Agency users.
Note: Here you can find the sitemap XML specification https://www.sitemaps.org/protocol.html
Also, you can use third-party XML generators (for example https://www.xml-sitemaps.com) to create sitemaps from the existing websites.