In reply to Dan Arkle:
I did once add extra filling to a sleeping bag. It was a lot of effort getting it in the bag (rather than all over the house) and measuring the amount going in.
I first searched around the bag to find the seam where it was originally filled from (running along the zip in my case) and opened up a slot about 4 cm long by unpicking the seam.
In the factory they use a sucker/blower to fill the bag, I didn't have access to such equipment so I made a roll of card that I could stuff with down with my hands inside the bag of down and then transfer to the sleeping bag without getting down everywhere by poking the tube into the slot I'd made and pushing the down out of the tube.
Hope this helps.