CPK is an archive format used in the portable versions of Persona games. It was developed by CRI Middleware and is the successor of CVM. All game data (except the executable) is stored in compressed CPK archives.
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
There are many ways to pack a CPK, but 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