MFWF, most of the so-called humanitarian flying (UN, Red Cross) is sub-contracted to commercial operators, so you need to find employment wiith one of them... I've been out of that scene a few years now, but a lot of these companies are (were?) based in South Africa, to name a few King Air Services, NorseAir, Naturelink, AirQuarius. Also Transafrik in Angola and Zimex, based in Switzerland. Don't have any contact details to hand but do a search on the Africa forum, should be helpful. Good luck and enjoy, it is some of the best flying you will ever do.
JD