Problem Enabling External Page Sitemap

I am trying to add a subdirectory of non-wordpress pagesto the sitemap. They are generated dynaically in /docs and cached copies exist at /docs-cache I do all this vis a plugin. I am scraping another site to get this info. All the pages render fine but I can’t get the BWP Sitemap to show these pages. I have implemented the following hook like so:

…`
add_filter(‘bwp_gxs_external_pages’, ‘starquestDocSync::integrate_sitemap’);
…
static function integrate_sitemap()
{
$html = file_get_html(‘https://docs.starquest.com/sitemap.shtml’);
$i=0;
$sitemap_locations = Array();

while($html->find(‘a’, $i) !== null)
{
$href = $html->find(‘a’, $i)->href;

if(stristr($href, ‘docs.starquest.com’) !== false) {
$href = str_replace(‘docs.starquest.com’, self::$base_url . ‘/docs’, $href);

$sitemap_locations[] = Array(
‘location’ => $href,
‘lastmod’ => date(‘c’),
‘frequency’ => ‘auto’,
‘priority’ => ‘1.0’
);
}
}

return $sitemap_locations;
}`

I have also checked the “Enable External Sitemaps” on the wp admin console. Yet no evidence of these files shows up. In the sitemap log the following message appears:

Requested sitemap (page_external.xml) was not found or not enabled.

Does this mean I have to add a module for the external sitemaps? I dont need db access to generate them, and I’d prefer to keep the code in my plugin. Is there something I’ve missed?

https://wordpress.org/plugins/bwp-google-xml-sitemaps/