I have to agree with Marcel. Even the best teleconverters are going to make the lens extremely difficult to use. I have a Nikkor 28-300 f.3.5-5.6 that I was hoping would work with a 1.7X-2.0X converter so that I wouldn't have to lug my Sigma 150-500 around with me when we flew cross-country to Yosemite just in case I needed it to capture a bird or bear or something. At 5.6 a 2X converter puts you somewhere between f/9-11, so you better have an amazingly bright subject and be willing to focus manually when you find the lens hunting - like I did.
If you need to extend the range then you need to go brighter. Save up for a f/2.8 lens, either fixed 300mm or zoom (I have the Sigma 120-300mm in mind) and that will work with the appropriate 2X. I know, I've tried it. Not cheap, but if you need that reach it comes at a cost. Until then, the Sigma I have is a great alternative, particularly with the DX body as the 500mm reach is what you'd get with a 750mm on a full frame camera.