Can someone go on a field mission at his or her own expense as a general helper/assistant? Can someone who happens to be travelling in the area in which a mission is working stop off and visit or perhaps lend a hand for a week or two?

No, the professional teams we send to the field cannot accommodate observers, untrained helpers or others who lack the qualifications and experience to be full participants in the work of the mission. All volunteers working in field projects must go through the normal recruitment procedure.