Will this teleconverter fits in Nikkor 55-300mm?

Kenko Teleplus MC7 DGX 2x Teleconverter - Nikon AF ~ Nikkor 55-300mm lens?

If it is Nikon mount then yes it will.

Just be aware that your f/5.6 max aperture at 300mm becomes f/11 with the 2x fitted.

The Kenko doublers will work on the 55-300 (wheras the Nikon made ones won't). This lens will however make your lens function only as a manual focus lens because of the reduction in aperture.