>I've never tried it, but I'd think a Kenko Pro 1.4x should >work.
If anyone wants to try that combination, PLEASE be careful.
As the other posters above pointed out, having the glass elements of the TC and lens come into contact will do neither any good! Using the Kenko Pro 300 1.4x on the 18-200mm Nikkor, the glass comes very close; the 18-300mm may not be so lucky.
I would go with the others and recommend not using a TC with this lens.