CCK field related to non-drupal table columns
Well, one of the possible ways to do that is to create a cck field of type integer and use "select list" as a widget. And then in your implementation of hook_form_alter you need to extract data from Customers table and populate the select with this data.

You might find the following piece of code useful:

function YOUR_MODULE_NAME_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if($form_id == 'YOUR_CONTENT_TYPE_NAME_node_form') {
    $sql = 'SELECT id, name FROM Customers';
    $result = db_query($sql);
    $options = '';
    while($row = db_fetch_array($result)) {
      $options .= $row['id'] . "|" . $row['name'] . "\n";
    }
    $form['#field_info']['field_YOUR_FIELD_NAME']['allowed_values'] = $options;
  }
}