WordPress MU subdomain redirection to domain

Hello,

I installed WordPress MU with domain name example.com. I have created subdomain demo.example.com through the WP MU admin interface. I purchased another domain – demo.com. My question is: how may I show content of the subdomain demo.example.com under the domain name demo.com ? I cannot use add-on domain in this case because WP MU doesn’t create folders of new websites.

regards,

Roman