Apart from the 30 kgs offered by Qatar Airways, I have 10 kgs that I will take with me. Is it possible to take these extra kilos in a second bag?
Airlines, including Qatari Airways, offer an excess baggage service, but it can be expensive.
Qatari Airways charge $50/kg for excess baggage from Europe to Africa ($40/kg if bought online ahead of time), although for flights to Africa you can get a bulk rate of $50/5kg (source:Qatari Airways)
Other charges may apply if your itinerary includes flights with other airlines, and the bulk rate is not available from all airports.
The Qatari web site suggests you call them to verify the rules and charges applying to your flights, although they say that if you have already booked a flight you should be able to buy excess baggage online.
Normally, both the option of paying extra weight or upgrading to business class can be costly.
There is usually a third option, if you have time, and things are properly planned. Carry it as cargo, as it is significantly cheaper. You pack it on boxes, and you carry it to the airport before the flight. It could be the day before, details vary from airline to airline, check with them.
If things are properly planned, the cargo will arrive in the same plane as you, otherwise it might mean going to the airport again to pick up the cargo. (I think it is the bulk rate the other answer is mentioning, I just noticed it now)
However, you would better check better the details of you travel package/tickets. If I am not wrong, you are allowed to carry 30kg + 7kg in your hand luggage that you take to the plane with you.
So at the end of the day, we are just talking about 3kg in excess. Even less if you are willing to trade sandals/sneakers for ticker boots or take an heavier coat to the cabin.
Hand Baggage allowance
In addition to the regular checked-in baggage allowance you may carry, you may also bring along hand baggage on board the cabin during your Qatar Airways flight:
Economy Class customers are allowed to carry one piece of baggage, not to exceed 7kg
On top of the hand luggage, ladies often also carry their purse, and men their computer.
With a bit of luck you might not pay more to bring the extra 10kg, except if officials are being more strict in a very packed flight.
In a similar situation, I brought 11kg in my hand luggage from Africa without being extra charged, however I do not endorse trying to do it.