Is it possible to load an external site map and merge it with the one generated by the plugin?
The items are loaded from this remote sitemap are merged with the one generated by the plugin.
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