How to log additional fields as standard?

[julioomoura]

I know we can log additional fields, using a parameter in the functions:
function (short_message, full_message, additionalFields, timestamp)

But I'd like to set an additional field at the initial configuration:

var logger = new graylog2.graylog({
    servers: [
        { 'host': '127.0.0.1', port: 12201 },
        { 'host': '127.0.0.2', port: 12201 }
    ],
    hostname: 'server.name', // the name of this host
                             // (optional, default: os.hostname())
    facility: 'Node.js',     // the facility for these log messages
                             // (optional, default: "Node.js")
    bufferSize: 1350         // max UDP packet size, should never exceed the
                             // MTU of your system (optional, default: 1400)
   ///////////////additional field
   "_additionalField": "content"
}); 

Is it possible?