Managing page access for custom dirs

[Alan]

Hello, it's possible I missed this part in the documentation somewhere but basically I would like to manage
which user groups are allowed to visit php pages that are located in 1 or more levels of directories.
CUrrently admin_pages.php only seems to be listing pages in the /users directory, and (custom) pages in the root directory, but not anything else. How do I make sure pages in -let's say- directory /xyz are also listed in order for me to be able to set user group access?

Thanks in advance.