PBPCat adds extra files into an EBOOT.PBP, and basically works as an alternative to bin2o in the way that it lets you store files within the eboot and allows you to read them back from within the program.
There is a small sample included, and the packer was compiled with mingw and only comes with a windows makefile. You should use the header files included with the sample to extract files.
This is a really simple program using the PBP file description found in YAPSPDoc.
http://forums.qj.net/f-psp-development-forum-11/t-releasec99-pbpcat-flatmush-117007.html