Don't do it!

A) the Canon teleconverters WILL NOT work. You risk trashing the rear element of the lens and mashing the teleconvertor

A third party option such as the sigma teleconverter may well work but..

B) The quality of your lens beyond 200mm on a DSLR is pretty poor. Ghosting. Flare. Softness. The teleconverter will magnify these flaws by either 1.4 or 2x

C) You will lose autofocus completely with the 2x convertor, you will lose autofocus for most of the zoom range with the 1.4x.

D) The depth of field will become so shallow at the effective focal length that it will be very very diffiuclt to accurately focus with the small dark viewfinder of the EOS400D

E) The lens will become potentially an effective 670mm with a widest aperture f11. So you will need to use fast shutter speeds to avoid shake. Fast shutters at f11 require fast ISOs, all of which degrades the image.

If you don't have the 75-300 already then don't get it.
If you have it aready consider getting rid and replacing it with something better.

This is before you consider teleconvertors.

If you want big reach then look at a 70-200 with a 1.4x or 2x. And it all has to be Canon L. The Sigma EX 70-200 f2.8 with sigmas 2x conv may be a decent option with faster max aperture for similar cash.

A similar costed option would be the sigma 50-500mm aka bigma. Not as fast but very good range and less messing about.

And as said elsewhere....

You need a monopod or tripod.

This may not be the advice you want, but it's the best advice I can give you.

If all of this is toomuch cash then look at the Canon 70-300 IS and forget the teleconverter.

If you want good tele images, you need to review your 75-300 decision.