Hi,
Well, supposing you create a custom user field for the user to enter his email address in, and some javascript code to auto fill the real email field that you would hide with CSS, it would be possible.
You would also have to rearrange the layout of the registration form to move the custom user field before the password email field because the custom user fields are normally displayed after the password field.
So it's possible but it will require a developer working a bit on it for you. Nothing complex but still a few lines of javascript, a bit of CSS and a view override.
If that's something you would like us to work on for you as a small custom development, we can give you a quote if you request it via our contact form:
www.hikashop.com/support/contact-us.html
But our partners, or any PHP/JS developer should be able to work on that too.