Not all modules are implemented yet in the 2.3 beta.
If you want to try Zentyal, have a look at Zentyal 2.2. That one will be able to be configured as slave to W2K8r2 Active Directory.
The Samba4 module in Zentyal 2.3 is still under heavy development, but when finished will do exactly what you want. It will even be able to replace Active Directory.