I do this a lot with nursery quilts need to be made into big child quilts, etc. Just measure well and make your front and back, either pieced or not, the size that you want. As mentioned above, put right sides together, front and back and stitch, like you would if you were doing a strip of sashing, etc. Cut your new batting the size you need and whip stitch (or zig zag if using a flat batting) to the edge of previous quilt. Press top and bottom pieces out carefully (don't want to flatten your batting). Baste and quilt and bind as usual.