build custom field in comments in drupal 6
In drupal 6 you should use hook_form_alter:

&lt;?php
function mymodule_form_alter(&amp;$form, &amp;$form_state, $form_id) {

if ($form_id == 'comment_form') {

$cid = 1; // TODO See bottom comment in submit funtcion about this.
$answer = db_result(db_query('SELECT answer FROM {my_module_comment} WHERE cid = %d', $cid));
$answer = $answer ? $answer : 2;

$options = array(
    '1' =&gt; t('Yes'),
    '0' =&gt; t('No'),
    '2' =&gt; t('Empty')
);
$form['answer'] = array(
    '#title' =&gt; t('Your Answer'),
    '#type' =&gt; 'radios',
    '#required' =&gt; true,
    '#options' =&gt; $options,
    '#weight' =&gt; -1,
    '#default_value' =&gt; $answer,
);
// $form['#submit'] = 'mymodule_comment_form_validate'; Uncomment and add function like submit, for validation your and other submitted data.
$form['#submit'] = 'mymodule_comment_form_submit';
}

function mymodule_comment_form_submit($form, &amp;$form_state) {
  $cid = 1; // TODO Here you should find CID - that will link your table record with Drupal comment Id record. Use print_r or devel dsm function for investigating structure of $form_state for seeking CID value. 
  $object = array('cid' =&gt; $cid, $form_state['values']['answer']);
  drupal_write_record('my_module_comment', $object); // You table have 2 columns: cid and answer columns.
}