FXS isn't capable of forwarding calls to an exchange. An FXS port connects to a single telephone and has no means to dial out, only to send ringing voltage to the phone if the extension assigned to the FXS is matched.
If your intent is to route certain exchanges to a conventional phone line or trunk you will need FXO ports. The regular TA900e has one such port allowing for a single talk path. You would need the TA924e-L2 version which has an additional eight FXO ports to accomplish this. With transparent proxy it's a bit tricky to intercept digit strings. I've used the emergency call routing function for this.
One issue that you will have is that calls routed out the local FXO ports will go direct (that's the point). Because of this any features based in the hosted softswitch on the SIP trunk won't function so transfer and conference features in the softswitch won't apply to locally-routed calls.
It would be better to build the FXO trunk and have the softswitch redirect the calls back to the TA900e, then push them out the trunk. This way the softswitch stays in the call and its features work. See the following for one way to do this: http://impulse.net/importing-offnet-numbers-to-broadsoft/