Multi sitemap

Hello
I have a little bit unusual situation.
There is the main website yurta.ro, which is built with wordpress and the yoast plugin perfectly generates the sitemap, but the website also has a subdirectory where the shop of the website (yurta.ro/shop) is located which is built with HTML. I’d like to add this too to the sitemap if possible.
Do you have a recommendation on how to do this?
Thanks

The page I need help with: [log in to see the link]