I've add both new and cannibalized down to various DIY Costco down throws. I use the "wet" method to put the extra down in the throw. Some have channels, some are modified KARO. I hang the throw by the edge in my walk-in attic, cut small slits along where the new edge of the modified throw will be, drop in the wet down, and pin the slits closed and ready for sewing later, after which I run the final product through the dryer.

NOTE: I rip out about two-thirds of the sewn-through seams, so have as few sewn-through seams as possible.