vinodvaswani

This plugin will give you ability to add related products of your choice in bulk. and you can also get the all related products of any product.
features include:
<ul>
<li>Add woocommerce related products in bulk. eg. select 5 related products form left side and select 2 products in right side, in results 5 selected related products will be 
related for each product in right side.</li>
<li>Select product category wise.</li>
<li>get all related products of any product.</li>
<li>Remove related product from any product.</li>
</ul>
NOTE: Woocommerce must be installed in you website.

WC Bulk Custom Add Related Products

This plugin will give you ability to add related products of your choice in bulk. and you can also get the all related products of any product.

Scott Nelle

Custom Related Products for WooCommerce lets you choose which products should show in the related products area on a product detail page instead of simply showing products from the same category. Take control of your detail views and show the products that you want!
How to use:
<ul>
<li>Edit any product. Under Linked Products you will now have a Related Products selector alongside the Cross-Sells and Upsells.</li>
<li>Select some products and save.</li>
<li>As long as your theme has a related products section, you’re done! By default all WooCommerce themes have this, so unless your theme specifically disables it you’re all set.</li>
<li>If you don’t select related products for any product, it will fall back to the WooCommerce default behavior of selecting random products by category. If you’d prefer to show no products you can configure that under WooCommerce > Custom Related Products.</li>
</ul>
This plugin has been tested with WooCommerce through version 10.

Custom Related Products for WooCommerce

Easy to use, But need a shortcode of the plugin.
Can you provide shortcode of the plugin?

Overall Good Plugin, Need plugin Shortcode

Custom Related Products for WooCommerce lets you choose which products should show in the related products area on a product detail page.

WC Bulk Custom Add Related Products vs Custom Related Products for WooCommerce