cpk is some sort of compressed format, and you'll need a cpk unpacker to open it.
Blame CriWare for making such a funky compression format. They offer some sort of free trial or something, but you actually have to contact them and give them a reason WHY you need their cpk unpacker. Or you can buy it (but it's WAY too expensive; never seen a program so expensive beside Photoshop).
You practically used the programs which the community uses to edit textures. If you can't with those, I don't think you should be texture hacking, unless you want to use Paint or something.