Security restrictions in a MoinMoin wiki work by the interface defined in the MoinMoin.security module. The Permissions class implements the basic interface for user permissions and system policy. If you want to define your own policy, inherit from that base class, so that when new permissions are defined, you get the defaults.
Then either assign your new class to SecurityPolicy in moin_config (and I mean the class, not an instance of it), or define your class directly in moin_config and name it SecurityPolicy.
For samples on using this mechanism, see SecurityPolicy.