Using the template_include filter in WordPress

gistfile1.php

<?php
add_filter( 'template_include', 'ja_template_check' );
function ja_template_check( $template ) {
	if ( is_category() ){
 
		// Get category information
		$cat = get_query_var( 'cat' );
		$category_info = get_category( $cat );
 
		// News sub-categories, where 7 is the ID for News
		if ( $category_info->parent == 7 ){
			return get_stylesheet_directory() . '/subcategory-news.php';
		}
	}
	return $template;
 
}