You just don't understand torrents well enough. First of all, if you have the first 17 episodes already, just toss them into the directory where the torrent is downloading, and it will count those as already downloaded. If that is not an option, most modern clients have the option of *not downloading* files that you don't want. Hence, if you wanted 18-24, you could mark 1-17 *do not download* and it will only grab 18-24.
-bmfrosty