Joao Amil
The UN does not employ their own pilots. They lease aircraft with crews from various operators who tender for the business on an ACMI basis. The crew are paid by the operator. We work for a South African company flying SA registered aircraft. You would have to obtain an SA licence or at least get a validation in order to fly them outside of SA borders. This a long drawn out process so your best bet would be to find out if any operators in your area (you don't say where you are from) operate aircraft for the UN and contact them.
Good Luck