Converting existing C/C++ code to PSP

I'm trying to research the PSP coding process and decide whether or not I think it would be possible for me to take existing C/C++ code for an application (nameless because I'm not even sure if it's possible for me so why say anything specific) assuming it's an open source software then the code is easy to come by.

I would very much like to port a portion of this software suite over to the PSP, or at least try to see if it's even possible but I'm not sure what is involved in altering code from standard C/C++ to PSP compatible C/C++.

Again, I'm not writing code, so tutorials aren't likely to be of help. I'm mostly just asking for basic info about how difficult or not it would be to convert existing code to PSP compatibility.

I think PMing the makers of Quake or Doom for psp would be most helpful for this dilemma

It can range from being very easy to incredibly hard pending on the amount of platform specific code in the codebase.