mymodule.install

<?php

/**
 * Import translations
 */
function hook_update_7001() {
  my_module_import_po_file('pt-pt');
}

function my_module_import_po_file($language) {
  $path = drupal_get_path('module', 'mymodule');
  $poFile = $path . '/translations/' . $language . '-mymodule-7.x-1.0.po';
  $file = new stdClass;
  $file->uri = $poFile;
  $file->filename = basename($poFile);
  _locale_import_po($file, $language, LOCALE_IMPORT_OVERWRITE, 'default');
}