I don't have an ATT HOME_CSC to compare it to, but looking at the U-Firmware, it sure looks like that's what you do. But I can't say for sure. I would hate to say that is true and mess up your data.
Here are the screenshots for the HOME_CSC and normal CSC for U-Firmware at the bottom as "Attached Thumbnails".
And BTW, You really don't need to recompress those files. Just use 7-Zip and you should be able to just remove the .ptt file and save and close the CSC file.
Good Luck and let us know how it goes.