Okay, the real answer to your question of how to do this is ... you are going to have to add it as a quilt as you go method.

Make a band of batting the width of the additional inches needed. For example I am going to say 2 inches. By band I mean measure the length you need from one side seam, all the way around to the same side seam. zig zag the ends together, not overlapping, but butted up against each other. Now, zig zag this band of batting to the bottom of your cover.
On the outside of your cover place the additional fabric, whether you choose to do more charm squares or a solid band, right sides together with your cover. This fabric should also be in a continuous band. On the inside of your cover place a band of your lining fabric, right sides together. Slide the band of batting you attached over the arm of your sewing machine. Stitch through the additional fabrics and your cover, 1/4" from the edge. Fold down (over the newly attached batting) both the outside and inside bands of fabric. Your cover should now be long enough. Maybe run a couple lines of stitching around the added band and then bind.

Hopefully this makes sense.