Supplementing a synthetic bag with down… can it be done?

If you are going to do that, you may as well get the materials you want and make it yourself. Synthetic bags probably won't have down proof material, especially if they are cheap. The material will also be much heavier than what you can buy cheaply. If you stuffed down in your bag, you then just have a non baffled 3lb sleeping bag good to 30 degrees. If you take all the insulation out, you are left with a heavy shell, no baffles, and extra insulation along the quilting lines. A sewn threw down bag will only be good to about 40* tops. If you are a beginning DIYer, consider the Ray Way quilt kit for your sleeping bag bag needs, its cheap and would be lighter and warmer than a Frankenstein bag. The instructions are perfectly clear and takes about 8 hours for your first quilt (probably less time and less frustrating than a franken-bag persuit). Another plus is that you can't get the materials as cheap as he sells them to you unless you buy bulk. Have fun with DIY, its more addictive than drugs or coffee

"Have fun with DIY, its more addictive than drugs or coffee"

Agreed.