How to create a new properties to $node (or $user and so on)
In conjunction with the Entity API module, you can use

hook_entity_property_info


  Allow modules to define metadata about entity properties.
  
  Modules providing properties for any entities defined in
  hook_entity_info() can implement this hook to provide metadata about
  this properties.


e.g:

/**
 * Implements hook_entity_property_info
 */
function MODULE_entity_property_info() {
  $info = array();
  $properties = &amp;$info['node']['properties'];

  $properties['nid'] = array(
    'label' =&gt; t("Content ID"), 
    'type' =&gt; 'integer', 
    'description' =&gt; t("The unique content ID."),
  );
  return $info;
}


See also entity_metadata_node_entity_property_info for default node property definitions.