The FS2 code is a very big codebase, I dont think anyone here can claim they know how all of it works. Im fairly knowledgable about the graphics system but know little about the rest.
First of all the code is really C with a few C++ features.
Someone who knows C really well is going to have a much easier time than a good C++ programmer.
Depending on which bits of the code you are looking at you have to know different things, WIN32, DX5/8, etc, though I imagine if you were to choose a job carefully you might just be able to program in ANSI C.
However where ever you are in the code you have to work with V's structures and functions. And since these are part of something so big it can be overwelming.
I would say you need to be fairly good at C/C++ to understand the code and very good to make any significant additions.
I suggest you download it and take a look. Doesnt do any harm to look. If you want to have a go at changing something start small.
Also this thread may be of interest to you:
http://www.hard-light.net/forums/index.php/topic,10563.0.html