Yes, you can use the routes option with your own custom function that will load and parse your external xml file, then inject its values as custom routes to the sitemap module.

See routes option: https://github.com/nuxt-community/sitemap-module#function-which-returns-a-promise

See an example here (it used the filter option, but a similar approach): https://cmty.app/nuxt/sitemap-module/issues/c56