If you add anything to the backing (other than original matching fabric), be sure that it will be ok to stay, as quilting will sometimes pull the fabric in, resulting in the added borders being quilted too.....depending on how your backing is pieced, I would maybe split it down the middle and add a strip there plus one on one end....making sure that enough of the added bottom border was placed over the quilt top so that it looked like it was supposed to be that way (you don't want just one inch of a different fabric to show)....clear as mud, right?