Programmically populate select list with hook_form_alter
I'm not sure to understand what do you want but here is a sample of code I've that may answer to your question.

$form['remise'] = array(
    '#type' => 'select',
     '#options' => array(
        0 => t('Sur les catégories & les marques'),
        1 => t('Sur les catégories uniquement'),
        2 => t('Sur les marques uniquement'),
     ),
    '#title' => t('Remises'),
    '#required' => TRUE,
    '#default_value' => 0,
  );