API access to custom attributes with restricted access

Hello, we use Personio API to access employee custom attributes.

There are some custom attributes located within a dedicated Personio data area with limited access rights, e.g. "special cost center".
Unfortunately we are not able to access these attributes within API employee calls („List Allowed Custom Attributes“, „List Allowed Attributes“).
What should be done to make ALL custom employee attributes available for API access?

Thank you!