Manual:$wgExtraNamespaces

Details
$wgExtraNamespaces is used to configure additional, custom namespaces for a wiki.

This is covered in detail on Manual:Using custom namespaces.

Do not use space in namespace names
Take care: When you define a namespace or set a name of a namespace to somthing which contains space character you should use underline inspite of it. For example 'My Namespace' is an invalid name and it migt provide you many problems and you should use 'My_Namespace' instead of it.