The Canon Extender EF 2x III does not fit because Canon’s teleconverters have a protruding front element and are designed for compatible telephoto lenses, not superzoom lenses like the Tamron 18-270mm.

A third-party teleconverter without that protruding design may physically mount, but this setup is generally not a good choice. With a slow superzoom lens, a 2x teleconverter greatly reduces the effective maximum aperture, so autofocus often will not work on a Rebel XSi, and image quality is likely to be poor.

So while a non-protruding teleconverter from Tamron or another maker might physically fit, it’s unlikely to give good results. The more practical solution is to use a longer lens instead of adding a teleconverter.