The simple answer is NO, i.e. you cannot 'cable' an external HD to the iPad docking station; the CCK (Camera Connection Kit) offers a SD card slot for use w/ a digital camera's storage care or a USB connection for the camera.
NOW, there are a number of portable Wi-Fi drives that either have their own storage (e.g. check the Seagate web site) or inexpensive devices that can accept a storage card - check out this forum
HERE and look at the 'stickies' on
Apotop Wi-Reader & AirStash (pics below) - I just bought the AirStash which allows me to use a SD camera card to transfer pics back & forth on my iPad using a Wi-Fi network (the link will explain the process) - movies & music can be loaded onto a SD card and streamed to the iPad; other options exist. Good luck in your choice.