No, Zeroshell does not implement a PPPoE server. It is only able to act as PPPoE client for DSL connections.
In any case, the integrated FreeRadius server answers to PAP requests and therefore you can use it to authenticate external PPPoE servers.
I think it is not so easy, because Zeroshell uses its templates to create the configuration files of FreeRadius. If you use another web interface you could confuse Zeroshell.
In the next releases I am going to improve the Radius’s web interface for a more generic use of the RADIUS service.