You can add the username field in the list of the user fields.
You need edit file /mod/customcert/element/userfield/classes/element.php , in this file you add username field in array $userfields.
For example after 'idnumber' you can add 'username'
you need add following line
'username' => get_user_field_name('username'),
So the complete array is
$userfields = array(
'firstname' => get_user_field_name('firstname'),
'lastname' => get_user_field_name('lastname'),
'email' => get_user_field_name('email'),
'city' => get_user_field_name('city'),
'country' => get_user_field_name('country'),
'url' => get_user_field_name('url'),
'icq' => get_user_field_name('icq'),
'skype' => get_user_field_name('skype'),
'aim' => get_user_field_name('aim'),
'yahoo' => get_user_field_name('yahoo'),
'msn' => get_user_field_name('msn'),
'idnumber' => get_user_field_name('idnumber'),
'username' => get_user_field_name('username'),
'institution' => get_user_field_name('institution'),
'department' => get_user_field_name('department'),
'phone1' => get_user_field_name('phone1'),
'phone2' => get_user_field_name('phone2'),
'address' => get_user_field_name('address')
);