Captchunder
Most of the so-called 'humanitarian flying' in Africa is done by private companies that are contracted in by UN, Red Cross etc.
A lot of these are South African ... some names to start with are Balmoral, Rossair, NAC and King Air Services ... there is also a Danish outfit called Aviation Assistance ... and a company called Transafrik which is based in Angola.
Good place to start would be the African forum ... do a search for 'humanitarian flying' ... the topic has been discussed quite a lot over the years.
You will do some great flying and meet fantastic people in some of the worst places this world has to offer
Good luck
JD