The MATLAB FDATool has a button that allows you to set Quantization Parameters for your filter. This is useful for determining how many bits are needed in coefficient storage. However, when I open the FDATool from the Xilinx Blockset, this feature is not available.
The Xilinx FDATool uses the Simulink FDATool, as opposed to the MATLAB FDATool (type FDATool at the MATLAB command prompt). The Simulink FDATool does not contain this feature.
If you want to use this feature of the FDATool, you should invoke it from MATLAB instead of Simulink. You can then store the coefficients to a variable that can be used in your Simulink System Generator model.