Is there way to use existing content type field to custom form in drupal?
In Drupal 7 you can use field_attach_form


function hook_form($form, &$form_state) {
  $form['fname'] = array(
    '#type' => 'textfield',
    '#title' => t('first name'),
  );
  $form['lname'] = array(
    '#type' => 'textfield',
    '#title' => t('last name'),
  );
  //attaching existing content type fields
  $node = new stdClass();
  $node->type = 'article'; //content type
  field_attach_form('node', $node, $form, $form_state); //it will attach all the fields
  unset($form['body']); //unset other fields like this.
  //..
  return $form;
}