Yes, it is, if you're using Shippo. I haven't yet had to do this, but it was covered in the recent Shippo seminar, which I believe is still posted on the Shippo discussion board here.
At least so far, Shippo's customer service/help features seem to be very good, so if you can't find the directions on the above board, just give them a call (the contact info is on their site, accessible once you've registered).
