YAMLFORM Element Settings
So In the end I had to use hook_form_alter, here's a quick trimmed down example of how I achieved my particular problem: 

function hook_form_alter(&amp;$form, FormStateInterface $form_state, $form_id) {

if ($form_state-&gt;getFormObject() instanceof YamlFormUiElementEditForm and hashandler) {

$form['newsection'] = [
  '#type' =&gt; 'fieldset',
  '#title' =&gt; t('New Section'),
];
$form['newsection']['newfield'] = [
  '#type' =&gt; 'textfield',
  '#title' =&gt; t('New field'),
];