Not directly using a cable, but there are drives that connect using wifi. Google for Seagate Wireless Plus or Corsair Voyager Air to see a couple of the many choices out there.
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.
I've merged the two threads you started on this subject as having two separate threads on the same subject can fragment the resulting discussion, making it difficult to follow, it is also against our rules.