gruntfile.js

module.exports = function(grunt) {
  
  grunt.initConfig({
    foo: {}
  });
  
  require('./plugin')(grunt);
  
  // It would be even easier if grunt had a 'use' method
  // grunt.use(plugin).use(another);
};

plugin.js

module.exports = function(grunt) {
  // Add them to the config object.
  grunt.config('bar', {});
  grunt.config('baz', {});
};