Manual:$wgExtraNamespaces

Details
Additional namespaces. If the namespaces defined in Language.php and Namespace.php are insufficient, you can create new ones here, for example, to import Help files in other languages.

Custom namespaces should start at 100 (to avoid conflicts with built-in namespaces) and stop at 255 (hard limit set by database, but removed 2005-05-02). Additionally they should follow the 'even numbers for main namespaces, odd numbers for associated talk pages' rule. Example: $wgExtraNamespaces = array(100 => "Hilfe",          101 => "Hilfe_Diskussion",           102 => "Aide",           103 => "Discussion_Aide"           );

When calling these custom namespaces with the getNamespace function, you must refer to them by number, not by NS_NAMESPACENAME. For instance,

getNamespace == 100

instead of

getNamespace == NS_HILFE

PLEASE NOTE: Once you delete a namespace, the pages in that namespace will no longer be accessible. If you rename it, then you can access them through the new namespace name.