functions.php

<?php
// Maps core part numbers to force sell after importing is finished.
add_action('pmxi_after_xml_import', 'danieldo_core_fees', 10, 1);

function danieldo_core_fees($post) {

    // Grab every product that was imported with a core fee
    $core_query = new WP_Query( array(
        'post_type' => 'product',
        'posts_per_page'   => -1,
        'meta_query' => array(
            array(
                'key'     => 'core_fee',
                'value'   => '',
                'compare' => '!=',
            ),
        ),
    ) );

    $products = $core_query->posts;

    foreach ($products as $product) {

        $core_id = get_post_meta($product->ID, 'core_fee', true);

        // Grab a post object by the core fee ID
        $core_fee = get_page_by_title( strval($core_id), OBJECT, 'product' );
                
        // Add the ID to a special custom field for the Force Sells plugin.
        add_post_meta($product->ID, '_force_sell_synced_ids', array($core_fee->ID), true);

        // Disable visibiity on core fees so they won't show up where they aren't supposed to by default.
        wp_set_object_terms( $core_fee->ID, array('exclude-from-catalog', 'exclude-from-search'), 'product_visibility', true );

        // Remove original custom field to keep things tidy
        delete_post_meta($product->ID, 'core_fee');
    }

} ?>