You can import the related products, categories, and tags using the default WooCommerce importer using a CSV by entering the IDs as comma-separated in the corresponding meta columns. The sample CSV columns for the related categories, products, and tags are as shown below:
|Meta: _crp_related_product_cats||Meta: _crp_related_ids||Meta: _crp_related_product_tags|
|106, 108||101, 102||115, 120|
You can insert the following code snippet into the functions.php file of the active child theme to import related products on the basis of ID.
Alternatively, if you need to import related products on the basis of SKU, refer here.