Add checkbox as a type in Woocommerce Product Compositons, not only radioboxes.
As far as I can tell you can only create composite products with radiobuttons, dropdown or thumbnails. This is great, but sometimes one would like to have just one product as optional, just like product-add ons has, but there you can't select another product like you can with composites. Should be pretty easy to include this option. Also if one mix composite and add-ons, the layout for totalprice and the way it's presented in the basket is messed up, not very streamlined, so it's not a good way to do it either.