When you start from scratch you can create an @icloud.com address and use it as your ID, but though it seems illogical you can't do what you ask. I don't know whether this is a technical issue.
However, I would suggest that having your published email address as your login is poor security. This used to be the case with MobileMe and there were quite a number of complaints from people who had found themselves locked out of their account because some idiot had tried to sign into their account several times without knowing the password. They then had to change the password in the ID and all their devices, and a couple of weeks later it would happen again.
This sort of nonsense sometimes arose through mis-typing, and sometimes because some idiot would become convinced that he owned the address and would keep trying to log in with it.
Of course, someone who knew your email address and felt malicious could render your account useless by keeping on doing this. I know all this sounds a bit extreme, and it is moderately unlikely, but these things do happen.
For best security, I would suggest creating a new free address somewhere with a garbage username which can't be guessed and is unlikely to be arrived at by accident. I know it's less convenient that logging in with your @icloud.com address, but you might like to think about the security issue here.
