Drupal adding an include
You can try module_load_include


  Loads a module include file.


Parameters

$type: The include file's type (file extension).

$module: The module to which the include file belongs.

$name: (optional) The base file name (without the $type extension). If omitted, $module is used; i.e., resulting in "$module.$type" by default.

Examples:

  // Load node.admin.inc from the node module.
  module_load_include('inc', 'node', 'node.admin');
  // Load content_types.inc from the node module.
  module_load_include('inc', 'node', 'content_types');