gistfile1.php

// system/modules/avisota-extend/config/database.sql
CREATE TABLE `tl_avisota_recipient` (
  `myfield` varchar(255) NOT NULL default '',
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

// system/modules/avisota-extend/dca/tl_avisota_recipient.php
<?php
$GLOBALS['TL_DCA']['tl_avisota_recipient']['palettes']['default'] .= ';{zusatzfelder_legend},myfield';
$GLOBALS['TL_DCA']['tl_avisota_recipient']['fields']['myfield'] = array(
  'label'                   => &$GLOBALS['TL_LANG']['tl_avisota_recipient']['myfield'],
  'exclude'                 => true,
  'search'                  => true,
  'sorting'                 => true,
  'flag'                    => 1,
  'inputType'               => 'text',
  'eval'                    => array(
    'tl_class'=>'w50',
    'maxlength'=>255,
    'importable'=>true,
    'exportable'=>true
  ),
);
?>

// system/modules/avisota-extend/languages/de/tl_avisota_recipient.php
<?php
$GLOBALS['TL_LANG']['tl_avisota_recipient']['myfield'] = array('Mein Zusatzfeld', 'Das tolle Zusatzfeld');
$GLOBALS['TL_LANG']['tl_avisota_recipient']['zusatzfelder_legend'] = 'Zusatzfelder';
?>