Hi,
That won't be possible as we don't know the modifications to allow that. It would require proper coding and testing.
Please understand that allowing registered email addresses to guest checkout with their email address will give them access to their addresses and orders by default.
This means that if I know your email address that you used for your user account on your website, I can guest checkout for myself with your email address and I'll have access to your address and your previous orders from the frontend user control panel.
So as this is a security issue or a potential problem for the identification of previous orders, we didn't implement the possibility.
Remember that as Xavier said, your customers will be able to guest checkout with the same email address as long as they didn't already created a user account on your website with that email address, in which case they can simply log on your website in order to proceed.