Extension tubes - Kenko ones do not work with EF-S lenses so will not work with your 18-55 lens. Nor do original Canon ones, but Jessops and Canon II extension tubes do support EF-S lenses so will work with your kit lens, for that reason I would recommend the cheaper Jessops ones.
Teleconvertors - the Kenko ones do not have protruding elements so will work with any lens, though they will only make their presence known if you are using a 10-pin lens, essentially the ones on mho's compatibility list or similar Sigma models.
This means that the aperture will correctly reflect the loss of light from using a teleconvertor, and the EXIF will include the correct focal length. If you use a 7-pin lens then everything will work okay but the EXIF and aperture values will be wrong. For example a 100mm f/4 lens with a 1.4x teleconvertor would actually become a 140mm f/5.6 but your shots will stay say they were taken at 100mm and you can still select an aperture of f/4 even though it will only let in the amount of light, and have the depth of field of, and f/5.6 lens.
One benefit to this though is that the 400D will only auto-focus with lens that have a maximum aperture of f/5.6 or greater. With your Tamron that would mean if the body was aware of a teleconvertor it would stop working as it would have a maximum aperture of f/6.3-8 with a 1.4x teleconvertor. Because with this lens the camera will not know you are using one it means auto-focus will continue to work.
The only thing to be aware of is that teleconvertors are essentially magnifying glasses between the lens and the sensor so they will also magnify any deficiencies in your lenses so they are only as good as what you use with them.
Kenko Pro 300 DG convertors are very popular because of their price and compatibility, and are only about £80 from Onestop Digital. I am very happy with my 1.4x version and intend on getting a 2x.
Michael.
