If the game you are thinking of uses less than 8 MB of RAM, can operate in 320x240 and fit on 128 MB SMC, and doesnt _need_ more buttons then the GP32 has (and is open source), and is written in a portable language like C/C++ then all thats required for a port to GP32 is to make (or use existing from the SDK if available) replacement functions for all of the platform specfic and external APIs. (assuming of course that the game will run on a 133mhz handheld)
Thats alot of IF's, but assuming you find a game that fits into that, then its still alot of work to write replacement functions for the renderer (because it probably uses GDI, DirectX or OpenGL) and for the audio, map the keys, etc..., even more so if you have to write custom functions to manage memory if the game uses more than 8 MB or too many keys, etc...
Not impossible, but _very_ time consuming, and we dont really have enough developers that could devote time to ports, most of the developers want (and rightly so) to write something new, something their own.
Its alot easier if the game was written with portablity in mind, like open source games on sourceforge, or games written for multiple platforms, etc...