The functionality still not in the roadmap yet.
You can do that if you have SMA (Security Management Appliance) since that appliance is intended to do centralise management and also reporting for multiple WSAs. Therefore if you make a change in SMA for WSA appliance, you can push out to all WSA appliances therefore all WSA has identical configuration.
If I remember correctly, the SMA will manage the policy configuration, but not the network or security config. You set up the System Admin, Network sections as normal for each box. Make the Security Services section match for the boxes, the SMA will tell you what doesn't match, but won't fix it for you (ENHANCEMENT REQUEST!!!!!), and then you can build the policies on the SMA and push them to the WSAs.