Adding zip to a non baffle down sleeping bag?

[flanagaj]

I have a really lightweight down bag that I use to add 10c to my main down bag.  The only issue is that although it's nice and light it has no zipper and I can't sleep in the damn thing due to the constraining effect mummy shaped bags have.  My plan is to put a zipper on the bag so that I can use it more as a quilt, but with the option to do up if I feel the need.

Any tips on how best to do this.  I was wondering whether there is a way of sewing the zipper on and then cutting the bag?