Not really sure how to repack, but for a personal project I used crifilesystem (you have to find it yourself).
At that point I had to manually reconstruct the cpk file.
This part was the most tedious part of the whole process, since the cpk could've used a number of different processes when it was made (file mode, data alignment).
Checking the differences between the two files in a hex editor also helped me. You'll want the files to be almost exactly the same. You'll notice big differences if made incorrectly.
You'll have to experiment with it yourself.
Hm, I see. Thanks a lot for all this info.