[Plugin: Google XML Sitemaps] Including lots of external pages in sitemap

Should have read the manual:

Adding other pages to the sitemap via other plugins

This plugin uses the action system of WordPress to allow other plugins
to add urls to the sitemap. Simply add your function with add_action to
the list and the plugin will execute yours every time the sitemap is build.
Use the static method “GetInstance” to get the generator and AddUrl method
to add your content.

Sample:

function your_pages() {
$generatorObject = &GoogleSitemapGenerator::GetInstance(); //Please note the “&” sign for PHP4!
if($generatorObject!=null) $generatorObject->AddUrl(“https://blog.uri/tags/hello/”,time(),”daily”,0.5);
}
add_action(“sm_buildmap”,”your_pages”);