As we are a company selling christmas articles, for us it could be very handy to add a whole product category to the upsell/related products.
For example: We sell a christmas tree, and to this christmastree we would like to show a clickthrough to the category christmasballs.
Is this possible in upselling or related products?
One more question: Can you also add more of the same related/cross products to more items in one go?
There is no way to do it out of the box. There are many 3-rd pary solutions (free and paid) on Magento Connect. Try them.