Hi,
Basically you can add "Additional fields" for user profile from admin panel.
Settings >> General Setting >> User Profile
Demo URL :
http://demo.x-cart.com/demo_goldplus/admin/configuration.php?option=User_Profiles
Here you can add select box with desired value.
However you should also read this, before implementing
http://convertable.com/blog/stop-asking-your-users-how-did-you-hear-about-us/
Best regards,