How do you turn a Mongoose document into a plain object?
Mongoose Models inherit from Documents, which have a toObject() method. I believe what you're looking for should be the result of doc.toObject().
http://mongoosejs.com/docs/api.html#document_Document-toObject
Another way to do this is to tell Mongoose that all you need is a plain JavaScript version of the returned doc by using lean() in the query chain. That way Mongoose skips the step of creating the full model instance and you directly get a doc you can modify:
MyModel.findOne().lean().exec(function(err, doc) { doc.addedProperty = 'foobar'; res.json(doc);});