Query “Category A” + 1 post from “Category B” – how?

I don’t see a way to do this with WP_Query alone. However…

function posts_where_add_post_wpse_96030($where) {
  global $wpdb;
  return $where." OR {$wpdb->posts}.ID = 1";
}
add_filter('posts_where','posts_where_add_post_wpse_96030');
$query = new WP_Query( 'cat=9' );

That will alter the query everywhere it runs so you will need to add conditions to the callback so that $where is only altered where you want it to be. For example…

function posts_where_add_post_wpse_96030($where) {
  global $wpdb;
  if (is_home()) {
    $where .= " OR {$wpdb->posts}.ID = 1";
  }
  return $where; 
}

Now the query will only be altered when it runs on a page where is_home is true. I don’t know what conditions you need. You didn’t include that in your question so you will have to work that out yourself.