Hi, John.
I think you can do this by using a resistor at the input. However, the noise gain will be higher and the stability will change because the gain of 1, created by the internal matched resistors.
You may also want to use a dual supply (for the AD8476) and use a lower input voltage that is within the IVR of -Vs+0.05V so that the low IVR of ADC will be reached.
You may also want to use ADI attenuators which you will find here - http://www.analog.com/en/rfif-components/rfif-attentuators-vga-filters/products/index.html
Regards,
Anna