Is it possible to add form elements to the existing fields with Field API? Is it possible solved with field api this task?
I need to add new elements to an existing field.
For example there is a field image. I create a widget and add a checkbox

/**
 * Implements hook_field_widget_form().
 */
function mymodule_field_widget_form(&amp;$form, &amp;$form_state, $field, $instance, $langcode, $items, $delta, $element) {
  $elements = image_field_widget_form($form, $form_state, $field, $instance, $langcode, $items, $delta, $element);
  foreach (element_children($elements) as $delta) {

    $elements[$delta]['#process'][] = 'custom_widget_process';

  }
  return $elements;




function custom_widget_process($element, &amp;$form_state, $form) {
  $element['myelement'] = array(
    '#type' =&gt; 'checkbox',
    '#title' =&gt; t('My checkbox.'),
  );
  return $element;
}




How do I save the value of the checkbox? (If I have a table for it)

Can I add additional column into an existing image field scheme?
(hook_field_schema_alter - not exist)
File Entity module allows you to add extra fields to file fields.


  File entity provides interfaces for managing files. It also extends the core file entity, allowing files to be fieldable, grouped into types, viewed (using display modes) and formatted using field formatters.