if ( function_exists('add_post_type_support') ) {
add_action('init', 'add_page_excerpts');
function add_page_excerpts()
{
add_post_type_support( 'page', 'excerpt' );
}
}
http://www.wpmayor.com/wordpress-hacks/add-an-excerpt-box-for-pages/(@source)
http://elearn.jp/wpman/function/add_post_type_support.html(@see)