I am not sure, but I think you can't use a teleconverter with that lens. Another problem: even if you could use it, your camera won't be able to use autofocus then, since the aperture decreases to a maximum of f8. Only the 5D Mark III and the 1D-camera's can still focus with f8.
Cameras: Canon EOS 7D, Canon EOS 40D
Lenses: Canon EF 17-40/4 L USM, Canon EF 24-105/4 L IS USM, Canon EF 70-200/2.8 L USM, Canon EF 300/4 L IS USM, Canon Extender 1.4x II
Want list: Canon EOS 1D Mark III, Canon EF 100/2.8 Macro L IS USM, Canon EF 500/4 L IS USM, Canon Extender 2.0x III