I'm afraid, here in support I can only instruct you on what is currently possible in the software today. If you would like any feature changed, please provide your suggestion in our Feature Suggestion forum.
There are only two options currently:
Your products are optionally associated where, what I described above, would be the only way to reference and prompt your users to buy the optional product.
Your products are required to be associated where, when the user buys x, they are forced to buy y.
From what I've read here, you're looking for both at the same time, which I'm afraid, is not possible.
If you are looking for an alternative, you could always create 2 products, one has your support/renewal baked in and the other does not. This way you can accomplish the above with separate products.