Adding additional fields to document before response

I have a request that I respond to as such:

function getstuff(req, res) {       
    var url = require('url').parse(req.url, true).query.u;

    Thing.findOne({url: url}, function (err, doc){    
        doc.newThings = 'some text!';
        res.json(doc);
    });
}

I'd like to be able to append some additional data to the document before I send it along to the client. The method I've tried above doesn't seem to be working. Is there clean way to accomplish this?