WordPress excerpt for specific posts in category

by

Modify the callback and check the post category:

function excerpt_read_more_link( $output ) {
    global $post;

    if ( in_category( 'news', $post ) )
        return $output . '<a href="'. get_permalink( $post->ID ) . '">more</a>';

    return $output;
}

And you should prefix the function name. In its current form it is not safe enough.