gistfile1.php

<?php
/**
* Implementation of hook_ctools_context_convert_list().
*/
function misc_ctools_context_convert_list($plugin, &$converters) {
  if ($plugin['name'] == 'node') {
    $converters = $converters + array('field_firstname' => t('First Name'), 'field_lastname' => t('Last Name'));  
  }  
  return $converters;
}

/**
 * Implementation of hook_ctools_context_converter_alter().
 */
function misc_ctools_context_converter_alter($context, $converter, &$value) {
  // Make sure we have a node to extract fields data from.
  if ($context->type != 'node' || empty($context->data)) {
    return;
  }
  $node = $context->data;
  $value = $node->{$converter}[0]['value'];
  return $value;
}

?>