Deiv:
You are wrong. The scope's front end will be explained in the doc that Digilent is planning on publishing later this summer. In it - you will find nothing external can improve the bandwidth.
As I said before - the Analog Discovery is a Digilent Product - these sorts of questions are better addressed by them. support@digilentinc.com.
The Discovery page:
http://www.digilentinc.com/Products/Detail.cfm?NavPath=2,1040,1043&Prod=ANALOG-DISCOVERY
has a datasheet:
http://www.digilentinc.com/data/products/analog-discovery/AnalogDiscovery-DS-2013-1.pdf
If your questions are not answered there - please ask Digilent for more information.
-Robin