Hi,

if the external system has links with the same domain of the main Joomla website you could merge 2 sitemaps in a single one. If domains are different it's mandatory that sitemaps will be submitted separetely.

Moreover this is not strictly needed because you can submit 2 sitemaps separately without any extra work or complex integrations between JSitemap and the external system.

If you want to develop a data source to fetch data from an external system, you need to learn about JSitemap API and get the SDK to develop it. In such case contact me at This e-mail address is being protected from spam bots, you need JavaScript enabled to view it

Cheers
J