Do you have an overhang of fabric and batting? that would make it easy to sew one on. Perhaps assemble the border as a sandwich and join it as you would a quilt as you go block. Perhaps google how to do that.
What I would do is remove binding and snip stitches back to about an inch from the edge. Fold the backing and top out of the way and add another piece of batting to each side and zig zag together flat sides first then top and bottom. Then I would stitch the extra strips of backing to the backing edges with batting and front moved out of the way and then sew on the front border adding sides first then top and bottom being careful to measure them accurately first. Then I'd lay the layers flat together again, press the seams lightly and quilt. Finish normally with the binding.
Hope this helps