Possible to add entire category as related?

Using the latest version (1.3.2), is it possible to set an entire category as "related" in the admin?

My client has about 3,000 products and multiple categories. When adding products, setting each related product individually is rather tedious, and they'd like to be able to set an entire category as related instead. I've hunted around for a module to do this and scoured the forums but am not finding anything directly related.