limit registrations by email provider

[Lpspider]

Okay, how would I limit registrations by email provider?

For example, dsmith@thissite.com - only emails @thissite.com can register on the forum. Additionally, how would I take the email address and transfer that into a username?

For example, dsmith@thissite.com registers. His username is automatically d smith.

So,

1) limit registrations by email provider
2) email address name is username
3) add a space after first letter of the email address

Is this possible? If so, how?

Thanks!