Is it illegal to open an email account and use someone else's name pretending to be that person?

NO it is NOT illegal.
There are a million "John Doe´s" and John Smiths out there.
If you use a different name than your own than you are just being smart by trying to hide your identity.
You cannot get into trouble so don t worry and don't listen to what other people say.

It all depends place you access the email. At work you could lose your job. Also look at a recent case pertaining to someone accessing Sarah Palin's email account, So yes it can be, depending on the intent.