This is possible with LanguageGetSpecialPageAliases hook.

Playing with hook in config file isn't that straight and comfortable like if there was simple variable like for other stuff, eg. namespace aliases etc.

I can imagine something like:

$wgSpecialPagesAliases = array(

"Wantedpages" => array( "Wanted articles", "WantedArticles", "Wantedarticles", "Wantedpages", "WantedPages")
)

or to avoid the necessity of repeated writing of the default set shipped with MediaWiki something like:

$wgSpecialPagesAliasesOverrides = array(

"Wantedpages" => array( "Wanted articles", "WantedArticles", "Wantedarticles")
)

which would actually prepend this defined set to the default one.