I wrote Siemens PTI support about this issue not too long ago, and the reply was that the recommended models for modelling this dynamically are the REGCAU/REECCU/REPCAU models, added as generator control, voltage control and auxiliary control for the renewable plant respectively. You can find descriptions of the models in the model documentation, and WECC have some pretty neat battery modelling guidelines to these models, which are easily found by a google search on "WECC battery modelling guidelines" for instance.
If you decide to use these renewable energy ("RE") models, the BESS should be modelled as a wind machine in the load flow, with Zsorce as @pssuser writes. You can set active power between +/- Pmax, and the dynamic model picks it up when initializing for dynamic simulations.
...but now I see that you are using PSSE v32 - are the "RE"-models included in v32?