add_field_to_dummy_cat.php

<?php
// Enter new field name
$fieldname = 'group';

// Get the SC processor instance
$processor = $catalog->processor;

// Get the IM instance
$imanager = $catalog->imanager;

// Get ItemMapper instance
$itemMapper = $imanager->getItemMapper();

// Get the dummy category
$dummy = $imanager->getCategoryMapper()->getCategory($processor->config->dummyCategoryId);

// If the dummy isn't null
if($dummy) {

    // Get the FieldMapper instance and initialize the fields of the current category
    $fiedMapper = new FieldMapper();
    $fiedMapper->init($dummy->id);

    if(!$fiedMapper->fieldNameExists($fieldname)) {

        // Enter new field data you want to add to your categories
        $customField = new Field($dummy->id);
        $customField->name =    $fieldname;
        $customField->label =   'My Group by field';
        $customField->type =    'text';
        $customField->position = null;
        $customField->default =  '';
        $customField->info =     'Put the group by value here (e.g. Group 1, or Group 2, etc.)';
        $customField->required = null;
        $customField->minimum =  null;
        $customField->maximum =  null;

        // Save the field
        if($customField->save()) {

            // Overwrite items, so that new 'group by' field can be used
            $itemMapper->init($dummy->id);
            if($itemMapper->items) {
                foreach($itemMapper->items as $item) {
                    $item->save();
                    // Save SimpleItem object
                    $itemMapper->alloc($dummy->id);
                    $itemMapper->simplify($item);
                    $itemMapper->save();
                }
                echo 'The category '.$dummy->name.' got a new '.$customField->name.' field.<br>';
            }
        }

    } else {
        echo 'The '.$dummy->name.' category already contains the '.$fieldname.' field.<br>';
    }
}