CPK

CPK is an archive format used in  the Persona series from Person 3 Portable onwards. It was developed by CRI Middleware and is the successor of CVM. Usually, most game data except for executables is stored in compressed CPK archives.

Mod Compendium
TGE's Mod Compendium can automatically merge multiple mods into a new CPK file. You can download it here and read this guide on how to use it.

CriPakGUI
CriPakGUI can open and extract most CPK files easily with its very user friendly interface. You can download it from here.

QuickBMS
aluigi's file extractor QuickBMS has scripts in .bms format for CPK archives. Run quickbms.exe, select cpk.bms, and then select your CPK file. It will automatically extract the contents to the directory of your choosing.

CRI Packed File Maker
While the Mod Compendium is an easier to use alternative, CRI Packed File Maker creates CPK files from an extracted directory and can be used to view file information such as data alignment, number of files, and compression modes. Use the commandline tool provided in the program's folder (cpkmakec.exe) to make a new CPK using the CSV file from the thread. cpkmakec.exe Usage: cpkmakec.exe data.csv data.cpk -dir=UNPACKEDFILESDIRECTORY -align=2048 -mode=FILENAME -view

To make your own CSV file for a different CPK, use CpkFilelistDumper.exe (also found in the thread). CpkFilelistDumper.exe Usage: CpkFilelistDumper.exe data.cpk