WPMultisite - Use subdomain for the main site

Can I in anyway force to use subdomain as my main site in WPMU installation? I know wp doesnt allow this.

This is my structure:

Main site: b.example.com
Subdomain of the above: sites.example.com
All http://www.example.com will be redirected to b.example.com

Kindly show me the way to do this if its possible.
In desperate to have it this way.

Thanks very much.