add_filter('body_class','additional_body_classes');

function additional_body_classes($classes) {
	$classes[] = 'class-i-need';
	$classes[] = 'another-additional-class';
	$classes[] = 'third-class';
	return $classes;
}

http://wpcanyon.com/tipsandtricks/adding-classes-to-body_class-in-wordpress/

Adding Classes to body_class() in WordPress

wordpress

PHP