jQuery serializeArray() add another value on top for passing to Ajax

var FormAttr = $('#form_id').serializeArray();

FormAttr.push({name: "Name_Of_Attribute", value:"Value_Of_Attributes"});

var data = $(form).serializeArray();
data.push({name: 'username', value: 'this is username'});

see also: jQuery post() with serialize and extra data

try

data[data.length] = { name: "username", value: "The Username" };