I quickly made a demo page for you to see how it is set up:
You create two input fields for the login that are set to Email as the Content Format. First one will be your standard input field where the user will enter the username/email
Next, create another Input element. Then, go to the Conditional tab, add the following conditions. In this screenshot, the Input Username or Email that you see in the condition is the field where the user will enter their username or email.
After that, be sure to uncheck the option This element is visible on page load and This input is disabled.
Finally, when you add your login button, go to your Workflow tab and set the Email field to your second Input element that is converting the username to an email.
