by default this isn't possible. the username is the username and that what you have to deal with.
BUT; you could try the following
though not 100% sure if this solution is really possible.
Hi,
The best way to provide sign-in/login functionality is to identify user with email instead of username, because emails are always unique.
Also there will be no security breach, if you check login in the microflow with email and password combination.
For the above, you can crate custom login page, call a nanoflow/microflow and check that user exist with entered email and password or not.
You can you default mendix sign-in system or create your own login api’s.