Configure a security-realm for LDAP (or others) described in [1] is not possible at this moment, there is neither an Operator configuration nor a way to use custom configuration due to the extra configuration of the security-realms that are required and how the server uses overlays to apply the user's custom configuration.

LDAP, ActiveDirectory or other security should be configured with the CR's

[1] https://access.redhat.com/documentation/en-us/red_hat_data_grid/8.3/html/data_grid_server_guide/security-realms#ldap-security-realms_security-realms