Hi I'm new here and I'm a photographer so I figured I pay my way forward some by sharing what I know in a few threads.
It depends on your budget. What is probably the best bet is to rent a real serious lens for the trip.
Just a few thoughts:
The lens you have is probably plastic and has an aperture of 5.6 or 6.3 on the long side. You will be able to shoot with this lens until twilight begins. At that point your shots will have no quality left in them. If you put a converter on this lens you will increase the focal length and the aperture by the same amount. In essence what will happen is that you have have very low quality shots even with the brightes sun of the day.
Your aperture is the flow rate in a lens. It is the size of the opening allowing light to pass through. You need X light to expose a sensor. As the aperture gets near 5.6 and so it really kills quality as the sun begins to fade.
If you rent a lens with a 2.8 or faster aperture, you will be shooting at the lowest ISO you can. When I shoot sports at 2.8 I can get shoots to sell even at 5 feet tall until the time when the sun hits the horizon. With a 5.6 lens I can only shoot till the beginning of twilight.
About tele-converters. They KILL image quality. Even with multi-thousand dollar lenses when you put a converter on you will get CA. That is when you can see a red or blue fringe of color on very bright objects. The biggest problem you will have is tan grass with tan animals and a high bright sun with no clouds. The edges of the objects in your shots will be very bad which really makes them mush together.
My best advice is to rent a lens like the 70-200 2.8 by most any maker. This is the best selling lens length and aperture and many companies do it well.
If you really want to get AMAZING shots, look for a 300, or 400 prime 2.8. These don't zoom. They are HUGE, but you shouldn't be very close to the animals so you will need reach. These can use teleconverters better than any plastic lenses, and many pros do use them.