Hi,
from your description it looks that what you need is to fetch data not from Joomla but from an external source and put them into the sitemap.
If so, the way to do this would be to develop a custom plugin using the JSitemap API to fetch data from your remote source and feed them to the JSitemap component.
Other than this you can use the type of data source 'Links data source' that allows to specify manually a list of links to be included in the sitemap.
If you need to get the JSitemap API SDK contact me at
This e-mail address is being protected from spam bots, you need JavaScript enabled to view it
Best regards
John D.