Inject non-acf fields during the rendering of acf-form

step_1.php

<?php
add_filter('acf/prepare_field', 'acf_prepare_field__inject_form_fields');
function acf_prepare_field__inject_form_fields( $field ) {
  if ( ! is_admin() ) {
    // look for the field AFTER where we want to inject something
    // I used a switch to make it easier to react to multiple insertion points
    switch ($field['key']) {
      case 'field_123a456b78c90': 
        acf_form_inject__excerpt();
        break;
      default:
        break;
    }
  }
  return $field;
}

// render our custom form field and display any existing data in case we are editing an existing post
function acf_form_inject__excerpt() {
  ?>
  <div class="acf-field acf-field-textarea" data-name="the_post_excerpt" data-type="textarea" data-key="the_post_excerpt">
    <div class="acf-label">
      <label for="acf-the_post_excerpt">Excerpt</label>
      <p class="description">Excerpts are optional hand-crafted summaries of your content that can be used in your theme.</p>
    </div>
    <div class="acf-input">
      <textarea id="the_post_excerpt" class="" name="jps[the_post_excerpt]" placeholder="" rows="8"><?php echo get_the_excerpt(); ?></textarea> </div>
  </div>
  <?php
}
?>

step_2.php

<?php
add_filter('acf/pre_save_post' , 'acf_pre_save_post__update_excerpt' );
function acf_pre_save_post__update_excerpt( $post_id ) {
    // bail early if editing in admin
    if( is_admin() ) {
      return $post_id;
    }

    // if our non-acf excerpt field has data
    if ( isset($_POST['jps']) && isset($_POST['jps']['the_post_excerpt']) ) {
      $the_post = get_post($post_id);
      $the_post->post_excerpt = $_POST['jps']['the_post_excerpt'];
      wp_update_post( $the_post );
    }

    return $post_id;
}
?>