locale_translation_projects_alter

function hook_locale_translation_projects_alter(&$projects) {

  // The translations are located at a custom translation sever.
  $projects['existing_project'] = array(
    'info' => array(
      'interface translation server pattern' => 'http://example.com/files/translations/%core/%project/%project-%version.%language.po',
    ),
  );
}