A .torrent
file is binary encoded, so you can't just parse out text from it. You would need to edit the file using a program to do it on your behalf (or I suppose you could edit the binary data by hand with a hex editor...).
The .torrent data structure would allow you to divvy up the data you want to download, but only on a file boundary (eg you could not break 1 file up over multiple torrents). This is because the hashes the torrent contains are computed from the the entirety of each files you are downloading. The .torrent file does not have the information necessary to generate hashes on just parts of the files to be downloaded (you could not compute the hash until you had already downloaded it). Additionally, Torrent clients do not have a means of joining together partial files on disk so you would have to find a way to cat the parts together.
You can remove or include files from the download, but that's really the extent of your capabilities, even if you decode, hack the torrent file, and either regenerate or re-encode the file.
If you were to write your own custom client, you could expose more functionality, but this is a completely non-trivial task.