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 something which contains a space character you should use underline inspite of it. For example 'My Namespace' is an invalid name and it might provide you many problems and you should use 'My_Namespace' instead of it.