If user declines 'gimme a site' and just gets a username can he get a site later

I want a user to be able to register, then have the option of getting their own site at https://whatever.whatever.com..

However, if someone just asks for a username originally, there appears to be no way to get a site at a later date.. am I missing something or is there a page I can direct the user to to signup for a site if he wants?