You cannot use fnServerParams with the current version of angular-datatables. I plan to do it soon.

For now, you can use the function DTOption.withFnServerData(fn) to override the ajax function.
You can find an example in the datatables directive.

You are now able to use fnServerParams.
For example:

$scope.dtOptions = DTOptionsBuilder.fromSource('yoursource.json')
  .withOption('fnServerParams', function ( aoData ) {
    aoData.push( { "name": "more_data", "value": "my_value" } );
  }

:D