I have a Montbell Super Spiral Down Hugger #3 (http://www.theultralighthiker.com/montbell/) which will take me comfortably just below freezing (-1C) but on my recent Everest Base Camp and Three Passes hike it was likely to get down to maybe -14C (at Gorek Shep) so I needed a little more warmth. I will be adding approximately 3 ounces of 900 fill power down to the bag. I also have a Montbell Superior Down vest, coat and trousers which I can wear (plus a down balaclava and down socks!) so I would be cosy enough.
Spot would like to come too, as you can see.
If you turn the bag inside out you can see where the baffles have been closed. It is a relatively easy task to push all the existing down to the bottoms of the baffle tubes, carefully rip the stitching which closes the baffles, then push handfuls of extra down into the tubes until you are satisfied they are full enough, then sew them up again.
You can buy 3 ounces (90 grams) of 900 fill power down for US$39.95 here: http://thru-hiker.com/materials/insulation.php or 800 fill power dry down here: http://www.tiergear.com.au/11/online-shop/duck-down-insulation-425g-15oz for A$30.80 This should drop the (comfort) temperature rating of your bag by approx 7C degrees.
On the inside of the bag you will find the seam where all the tubes have been sewn closed. You have to unpick a section at a time of stitching adjacent to the tube you want to fill.
Like this. A stitch unpicker is a good idea. So is a wife with superior manual dexterity.
This has exposed one side of the tube. You will have to unpick the stitching on the baffle material to expose the other side of the tube. You need to make opening large enough to fit your closed fist (full of down)
Opening up the tube.
Here it is all sewn up again. Two lots of restitching are necessary, one to close the baffle, the next to close the tube again.
Hope this is clear enough. Some additional useful instructions here: http://www.doityourself.com/stry/how-to-replace-feathers-in-a-down-sleeping-bag See videos here: https://www.youtube.com/watch?v=ZRMJUZFTnHM & here: https://www.youtube.com/watch?v=sCJp2C4EFjU