Manual:$wgNamespacesToBeSearchedDefault/ja

詳細
The variable holds an array of namespaces, indicating which namespaces are enabled for searching by default. The array is indexed by the namespace constant. Namespaces not added will not be searched by default.

名前空間の追加
To add other namespaces you just add this to you "LocalSettings.php" file, e. g. for the project namespace:

Changing options for existing users
In MediaWiki 1.16 and later, you do not have to change options for existing users.