to add to what John has said, using a Teleconverter will increase the effective range of the lenses
With a 1.4 tc your 18-55 will become a 25-77, and your 55-200 a 77-280.
However, the max aperture will also decrease by 1 stop (so the 18-55 will end up as a 25-77 f5.6-9.5) and you will lose some quality.
You would be better off getting a 70-300 zoom, which will give better range, max aperture and IQ than the 55-200 + 1.4x tc.
The 2x tc gives greater increase, but the 2 stop penalty, and greater loss of IQ would make it virtually unusable.
Where teleconverters are useful is when you have a reasonable long wide aperture lens - such as a 300 f/2.8 prime - the (relativity) modest cost of a good 1.4 tc will give a 420 f/4 for a fraction of the cost of a 400 f/4.5 prime.