You can use a microflow to do that. Give the microflow an input string or an object that contains the account name and do a retrieve on the database on the System.User object using the following xpath [System.User/Name = $inputName] where $inputName is the name you want to check.
By default mendix already does this check if you commit a user object. This ensures that no duplicate usernames will excist. However i can imagine that you would like to do this check on the onChange of your new user screen giving the user feedback right away.