I'm sorry but from my point ofview this shouldn't be possible, once P-code was actually created to protect the original code from being seen. However, you can talk to the authors of the code and (gently) ask them to compile function versions of parts of their code to P-code, so you will be able to use them as functions in your program.
You will still not know how they do what they do, but you can have the results you want.
Unless it is one's own pcode file and the file as been lost, seeking such capability is unethical. For me it is equivalent to plagiarism, copyright infringement and IP stealing. The file was pcoded indicates that the author wishes to copyright protect it. Of course it may also be the case that the author copied it from someone else and wants to hide the copied code. This would also be unethical.