Right now, you will need to use the SeamClient for that. SeamClient is currently experimental and I'm pretty sure its design will change in the near future. Ultimately, I'd like to get rid of it and roll this functionality in so that you don't need to do anything different.
For now, you can try out the SeamClient, which just extends JSFClientSession. You can download it from the JBoss Maven repo here:
Currently, you will see a stack trace on the server side, but it doesn't do any harm. That should at least make conversation scope work in your tests until we get Seam support fully completed.