WCF/WF - KnownType on external interface

I'm new to WCF. I'm using it for the first time to package a Business Logic Layer, making it usable over the network. In my business dll (class library) there are several interfaces and derived classes. I read about KnownTypeAttribute, and I understand how it works when you declare the data contract in the same project of the WCF service, but I haven't quite grasped how to specify a data contract for an already existing type which is packaged in a referenced library. Any help would be appreciated.

You can use Add service reference the proxy creates partial data contracts. You can modify the generated data contract by visual studio utility but it will overriden when its referenced again.

Thank you. I decided to use a concrete type for one of the members of the class that was generating a deserialization problem. It works fine now.
Cheers