There currently isn't a migration tool that will move the User information and messages from an Exchange backed setup to a Domino back end setup. There currently isn't such a tool planned - carting messages over would be just about impossible. It may be possible to bring user information over but since we can't create accounts on the fly in the DUC interface the alias (mail nickname in Exchange) would have to match up to the short name for users in Domino if that were to have any chance of working.
Either way, it's not something we have in place today and it's not committed as anything we plan to support down the road at the moment.
