There is no such thing as "their domains". A user on the server is a user for ALL of that servers virtual domains.
Example for my server: user@brummell.net, user@lewcarlaw-autobody.com, user@pounder.kicks-ass.net and user@sentrycommunications.ca all go to an account (or pseudonym) called user.
Now if you mean having user@domain.com and user@domain2.com going to two seperate accounts, I doubt that will ever happen, that's not how Mitel intends for the distribution to be used so it's unlikely they will add it. Now, if a developer wants to make an add-on, that's a different story...