Not that I've actually tried it but in Weld both Conversation and ConversationManager are injectables so if you have a clear picture on how to do stuff, go alternative.
what do you mean exactly by
How bout adding addConversation method to the ConversationManager API?
When you have an embedded LRC (mini-usecase within the usecase) then you're in effect adding a conversation. Or I guess suspending the active LRC for the LRC associated with the mini-usecase. Which seems to be similar to the nesting concept from Seam 1/2.
But then there's the issue of how to handle the SMPC with nesting.