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?