I've always wanted to cross seed files; however, I found that most guides online were made with uTorrent as the model client. Thus, I was left to screw around with Transmission and figure things out for myself. Having done that for a while, I finally figured out the trick. What follows is a tutorial that I hope can be of use to people who want to cross seed with Transmission as their chosen torrent client.

Let's say you downloaded a torrent file from TPB and you want to seed it to IPT.
1. Download a torrent file from TPB. Double click it. It will look like this:

2. Click Add. The file will save to your Downloads folder. Once the download if finished, it will look like this:

3. Download a similar .torrent file from IPT.
4. Double click the .torrent file.
5. The window will open like so:

6. Do not click Add
7. Rather, take note of the discrepancies between this torrent's FOLDER NAME, FILE NAME, and CONTENTS and the one you downloaded previously
8. ALTER the folder name, file name, and contents of the TPB torrent to match the folder name, file name, and contents of the torrent downloaded from IPT.
9. So originally you had this:

10. And now you have this:

11. Double click the IPT .torrent file again.
12. Click Verify Local Data. Click ADD
13. Congrats, the torrent will now start seeding to IPT

Source: N/A