No. These adapters were made for lenses with an aperture of 2.8.
If you use this adapter, your light will be halved -> you will then have to multiply the 100-300 f4.5-5.6 by 2. This means that your camera will no longer be able to focus. They can only focus up to f/8 at the most.
Better buy a 150-600mm from Tamron or Sigma... the cheapest costs 900.- €.