I would like to have one more piece of luggage travelling from Dusseldorf to Yaounde, Cameroon; what to do? I will travel on 29 September, and I'm in Bielefeld, Germany
I'am assuming you haven't reached the maximum baggage allowance. I would go to the airlines website, log in to "My Trips" or something similar and then add an extra bag to your reservation. Be sure to add the bag for both ways of your trip.
Alternatively you can also do this at the Airport but charges will most likely be higher.
Depending on the airline, this will cost you from 20 $ to 300 $ (in my experience).