Pfidy,
This is one of my complaints too as this type of thing is available in XC4. Please read my idea for "fixing" the "Force customer to choose product options before adding a product to cart". Your need would also solve my problem.
http://ideas.x-cart.com/forums/229428-x-cart-5-x/suggestions/6653302--force-customer-to-choose-product-options-before-a