No point in trying to use them for the UPK packer, like i said before:
You can't repack the files.
First of all the Texture2D from UDK is TGA while the one in the UPK Packer is "incomplete" and compressed Texture2D file.
And second. It's not cause of the "Security" to make sure it can't be mod'ed, but cause how the Unreal Engine works.
Even you found a way to repack it so would you still unable to change any files cause you don't have the "compress/decompress-coder" to recive or update the streamkeys, hexkey data and series numbers, even by changing the alpha channel in the texture is enought to make the GPK corrupt, cause you added extra data that you didn't update add to the "list"
If you read the README so does it even say that the UPK packer doesn't even support Add or removing of file.
It is thanks to this feature of insane compression and handling of the files that we are being able to run this game in max graphic.
Without this feature so would we have 5 FPS even at medium graphics.
It is not like all other non-Unreal Engine games where you can just exract and repack the files without any problems.
