Any teleconverter will convert any lens to a longer focal length of lower quality. A 1.4x extender with the 400/5.6 will give you a 560mm f8 lens which, without autofocus and IS is not a particularly attractive option in my view.
Nevertheless, with lens on a sturdy tripod and a clear and contrasty target, you should be able to capture a more detailed image than you would without the converter.