It's possible but it takes a number of customizations. The WordPress user table requires a username, so what you have to do is replace that with the user's email address. Since WordPress does not allow usernames to change, you also need to also account for when the user updates their email address.
There is a prewritten tutorial on how to set this up available to premium support subscribers. However, all of the filters you would need to do it are freely documented in the plugin's documentation.
Incidentally, with the upcoming 2.9 release, the updates to the form building function will make the process much easier. There will also be an extension available to implement this customization so no coding would be required. That will be avialable to premium support subscribers.