Subdomains

I have installed MU into blog.mydomain.com therefore by default users will be creating blogs on
userchosenname.blog.mydomain.com. This is not the url we want users to see when they access their blog, we want them to use

userchosenname.mydomain.com

It is not possible for us to buy a new domain name to host our blog as a separate entity to our main domain. Because of the server set up I cannot install in the root so I had no choice but to install on a subdomain.

Is there a way to use htaccess to send all blog owners to userchosenname.mydomain.com once they have signed up and want to come back to their blog?