After I had completed the script `parse-nodejs-express-body-by-hand.js` I sort out was possible get the same result using the `qs` library :D

output.js

// before
{ 'content[title]': 'test',
  'content[excerpt]': 'test',
  'content[content]': 'test',
  'content[featuredImage][title]': 'test',
  'content[source][title]': 'test',
  'content[source][url]': 'http://google.com',
  'content[source][author]': 'test',
  'content[source][publishedAt]': '25/07/2014',
  'content[topics]': '539f11bcfdf15258000d9922' }

// after:  
{ content:
   { title: 'test',
     excerpt: 'test',
     content: 'test',
     featuredImage: { title: 'test' },
     source:
      { title: 'test',
        url: 'http://google.com',
        author: 'test',
        publishedAt: '25/07/2014' },
     topics: '539f11bcfdf15258000d9922' } }

parse-nodejs-express-body-by-hand.js

function parse (req, res, next) {
  var body = {};
  Object.getOwnPropertyNames(req.body).forEach(function(name) {
    var value = req.body[name],
        baseField = getBaseFieldName(name),
        fields = getInnerFieldsNames(name),
        obj = {},
        prev,
        prop;

    if (!body[baseField]) body[baseField] = {};

    for (var i = fields.length - 1; i >= 0; i--) {
      prop = fields[i];
      if (i === 0) {
        obj = prev || value;
      }
      else if (i === fields.length - 1) {
        obj[prop] = value;
        prev = obj;
      } else {
        obj[prop] = prev;
        prev = obj;
      }
    }

    body[baseField][prop] = obj;
  });

  req.body = body;

  next();
}

function getBaseFieldName (fieldName) {
  return fieldName.match(/^([^\[]*)/)[1];
}

function getInnerFieldsNames (fieldName) {
  var regex = /\[([^\]]*)\]/g;
  var matches, output = [];
  while (matches = regex.exec(fieldName)) {
    output.push(matches[1]);
  }
  return output;
}

parse-nodejs-express-body-qs.js

function parse (req, res, next) {
  require('qs').parse(req.body);
  next();
}