As has been said what you want is an extender (also called a teleconvertor or TC). There are two magnifications available, 1.4x and 2x, with the 1.4x you will loose one stop on the max aperture making your lens (which I assume is f5.6 at the long end) into a 420mm f8 lens. Not sure what camera you are using but with a Canon you will loose autofocus (except for with the 1Ds and the 5D3). The TC will also impact on the image quality so you might find that you are better cropping harder rather than using one. If you are shooting with a Canon then forget the Canon TCs as they will not fit your lens, look instead for the Kenko as suggested above. A used Kenko 1.4x will set you back ~£80-100 or if you can find the older 1.5x it will be a bit less (~£60) and may attempt to AF. Avoid getting a 2x as they only work well on the big prime lenses.
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