Users can be added via Operator with an identity.yaml described in the Operator Guide [1]
But this requires a full cluster restart to take effect.

As an administrator I would be able to add user credentials by adding it on the Operator level and it should take effect without restarting the cluster to prevent side effects, burden the cluster with re-balancing or possible data loss.

 [1]https://access.redhat.com/documentation/en-us/red_hat_data_grid/8.3/html-single/data_grid_operator_guide/index#adding-credentials_authn