Hi,
HCP does not have a user store. By default the user store (or rather IDP ) used (see Trust in cockpit), is SAP ID service (account.sap.com). But you can configure HCP to use your own user store as long as it can speak SAML. If you have an on-premise SAML speaking IDP (like MS's Active Directory Server or SAP Single Sign-on), you can configure it as your account's IDP in the Trust section of cockpit. This is described in documentation.
Today, you cannot directly use the user store in ABAP (or other servers) from HCP.
Regards,
Dinu