Volunteer Visaというのがあった 就労ビザよりこっちのほうが取りやすいのかな
http://www.truetravellers.org/volunteer-visa-in-ghana/
http://www.ghanaembassy.org/index.php?page=visas
http://www.ghanaembassy.nl/index.php/consular-section/visa-applications.html
http://www.ghanaembassy.dk/content/visa-application