My daughter got this hand-me down coat that only has buttons. Its poofy and warm, but the buttons don't really work well. They come unfastened easily. I want to add a zipper to it so it will stay closed, but I'm not sure if it will work. I think it would mean the stitching would show through to the other side, but I think that will be ok as long as I do a good job. I would just leave the buttons on and use them in addition to the zipper. Any thoughts?