you could try picking up stitches like you did to make the cuffs and then knit in rib till you get the width you want cast off loosely and stitch it down with the yarn. There is a way to knit into the back of the existing bit to join them but it is a bit complicated to explain via post...you really need to see it IYGWIM. Stitiching it down would be just as good. A word of warning though don't try to unpick from the cast on end and join it in that way...you will just end up with a big mess that way!