How to Edit a contributed module
You should never edit core or contributed modules if it can be helped (which it almost always can).  I haven't used Feedback, but the general Drupal way of extending forms is to use hook_form_alter from your own custom module. 

Basically create a simple module.  All it needs is a mymodule.info file and a mymodule.php file inside of a folder in /sites/all/modules. Here is some more info on creating a module.

Then in your mymodule.php file, add a function that looks something like this:

/**
 * Implements hook_form_alter().
 */
function mymodule_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'FEEDBACK_MODULE_FORMID') {
    $form['custom_email'] = array(
      '#type' =&gt; 'textfield',
      '#title' =&gt; 'Email',
      '#required' =&gt; TRUE,
    );

    $form['#submit'][] = '_mymodule_feedback_form_submit';
  }
}


This will add a new field to the form, and register a submit callback for the form.  

Finally write the _mymodule_feedback_form_submit function to actually do something with the value when the form is submitted.