Register form questions with buddypress installed

Hello,
In my WPMU network, with buddypress installed, I use the default registration form - wordpress register+buddypress register. What I want to do is
- create a subsite(blog) option is always CHECKED as default (currently it is unchecked in the registration form)
- default subdomain name is the same as username. (well, not vey important)

How to do these?