Manual:$wgExtraNamespaces

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

This is covered in detail on Manual:Using custom namespaces. Note that the first custom namespace should use the ID 100, as IDs from 0 to 99 are reserved (and negative IDs have a special meaning). The maximum namespace ID is 255.

Example
To add a 'portal' namespace to your wiki you need to create 2 new namespaces (one for the portal page and one for its associated talk page: