Pluggable System

I'll give a brief details about a system I'm currently building.
This application is using plug-ins to provide service and connect to partner's automated services through socket connections.
Using System.AddIn.

I've created two major type of plug-ins:
1. Front-end: add-ins that used for creating front-end interface for other applications through socket connection. (ex. Web Services[self-hosted through WCF], free-formatted data, etc.)
2. Back-end: add-ins used for creating connections to our partner's automated services through their specified protocol.

The problem is; how can I make the front-end add-in to call a method from the Host-Application with the required parameters then route the data to the right back-end add-in?

Thanks.