Adding service behaviour into MS Workflow

Basically you use an IServiceBehavior to add the extension and a BehaviorExtensionElement to add the IServiceBehavior.

public class StringWriterBehavior : IServiceBehavior
{
    public void ApplyDispatchBehavior(ServiceDescription serviceDescription, ServiceHostBase serviceHostBase)
    {
        var host = (WorkflowServiceHost)serviceHostBase;
        host.WorkflowExtensions.Add<TextWriter>(() => new StringWriter());
    }
}

public class StringWriterElement : BehaviorExtensionElement
{
    public override Type BehaviorType
    {
        get { return typeof(StringWriterBehavior); }
    }

    protected override object CreateBehavior()
    {
        return new StringWriterBehavior();
    }
}

And the config file:

<system.serviceModel>
  <extensions>
    <behaviorExtensions>
      <add name=“stringWriter“
           type="OrderService.StringWriterElement, MyWorkflowService"/>
    </behaviorExtensions>
  </extensions>
  <services>
    <service name="OrderWorkflow“
             behaviorConfiguration="OrderWorkflowBehavior">
    </service>
  </services>
  <behaviors>
    <serviceBehaviors>
      <behavior name="OrderWorkflowBehavior">
        <serviceMetadata httpGetEnabled="True"/>
        <stringWriter />
      </behavior>
    </serviceBehaviors>
  </behaviors>
</system.serviceModel>

source:
http://social.msdn.microsoft.com/Forums/en-US/wfprerelease/thread/a9b45eaf-c8e2-444c-819d-e448868e68bb/