You should be able to use variation control to do that. The following URL should help:
The WP eStore uses an express checkout model to minimize hassles for customers when they buy a product (http://www.tipsandtricks-hq.com/ecommerce/wordpress-ecommerce-knowledgebase-523#express_checkout) so it doesn’t have the option to force a customer to register before they can buy. But it can be tweaked so it forces the customers to log in/register as a user on your WordPress system before they can make a purchase.