Domain redirection

How i can do to enable user to use their own domain name to view a blog?
ex.
i have WPMU as http://example.it
and user with a blog http://user.example.it

the user by a domain (for example myowndomain.it) and want use that domain to view his blog instead subdomain address http://user.example.it

how can i do to set WPMU to allow that?

Thanks

Michele