Here are some interesting facts about teleconverters:
1. Teleconverters accomplish their task through the use of optics to further magnify the object to be photographed. This means you are inserting more optical interfaces between the object to be photographed and the film in your camera. Each optical interface that you insert in the light path has the capability of reducing image quality - NO teleconverter can improve the image quality that was possible with the lens alone at a shorter distance, it can only enlarge the image and degrade the image to some extent.
2. As the focal length increases, so does the apparent 'camera shake' associated with the longer focal length - you will need to take this into account by setting your shutter to a higher speed or resort to using a tripod to reduce the effect.
3. The minimum focusing distance that was possible with the lens alone changes minimally. If your 100mm lens alone will focus down to 18 inches and you attach a 2X teleconverter, you now effectively have a 200mm lens that focuses down to 18 inches + the physical length of the teleconverter.
4. The amount of light entering the camera body through the lens-teleconverter combination is reduced. In the case of your 100mm/4 lens and a 2X teleconverter, your effective lens is now a 200mm/8. Since less light (actually ¼ as much) is entering the camera, you must compensate for it. In SLR cameras, the meter reads the light entering the body, so proper exposure is still obtained by reading your camera meter as always. However, less light means that exposure times may need to be increased through the use of slower shutter speeds - again requiring the use of a tripod and perhaps a remote shutter release. You may have to switch to a faster film to offset this effect also.
5. In general, teleconverters are designed for short (200mm and less) or long (300mm and greater) lenses and can not be switched back and forth. In general, you can expect more image degradation with a 2X teleconverter than with a 1.4X teleconverter. In general, 3X teleconverters are nearly worthless because of image degradation.
6. Always read your lens specification sheet before attaching a teleconverter - damage may occur otherwise.
What TC do you have? Nikon doesn't recommend use of theirs in anything wider than 70-80mm, but I think that's for 35mm (full frame).