You should be all right if you sew that extra batting strip to what's already there. I've done it many times when I miscalculated (OK, I botched the batting layout in my eagerness to get the quilting done). I just sew the new batting onto the existing with a herringbone stitch--or some other stitch about 2 inches apart. Once it's quilted, you'd never know because the quilting holds the new piece of batting securely in place. Just be certain to use the same batting as in the rest of the quilt.