If I understand your question correctly, you want to write a module that hooks into the whois module to alter the returned content. 

The general method has been described here: http://drupal.stackexchange.com/questions/73880/hooking-into-a-contrib-module

Unfortunately, the whois module doesn't provide a hook. 

However, you can change the output through preprocessing, for example in your theme's template.php file, or in your custom module (see also: https://api.drupal.org/api/drupal/modules!system!theme.api.php/function/hook_preprocess_HOOK/7):

    function yourthemeormodule_preprocess_block(&$variables, $hook) {

      if (isset($variables['elements']['whois_lookup_data'])) {

         $whois_modified = yourtheme_modify_whois($variables['elements']['whois_lookup_data']['#children']);
         $variables['elements']['whois_lookup_data']['#children'] = $whois_modified;
         $variables['content'] = $variables['elements']['whois_lookup_data']['#children'];

      }

    } 

    function yourthemeormodule_modify_whois($content) {
       // do what you want to do to modify the output
       return $altered_content;
    }