How to extract list of pages from wordpress domain (subfolder) to main site page?

Question

Create a sitemap

The sitemap is on the main domain and contains a list of static pages. To this list I want to add a list of pages from the subdomain (in the wordpress subfolder).
The problem is that the wp_list_pages function does not detect these pages (although I successfully found the blog pages on this subdomain using this function)
Which function will help me? Where to dig?