Hello,
This is possible but it requires a customization of the button code we provide you, it's not a supported option within our system itself.
As a very basic overview, we place a unique link for each of your products within each different Add to Cart button. An experienced web developer could take those links and build them into a drop down menu so that the corresponding product is added to the shopping cart when it is selected in the menu instead of the normal method which would be to list a series of buttons for each product.
To find someone who can assist you with this kind of extra customization for your site we can recommend the developers listed here:
http://www.e-junkie.com/ej/developer-directory.htm