Picking up extra stitches is what the author of SKS recommends. I've also seen a way to pick up the stitches at the gusset in such a way that there isn't a hole. Here's what a quick google search turned up from this
"Sitting back and admiring your work thus far, you've probably noticed a chain of stitches running up each side edge of the heel flap. These stitches are to be picked up and worked into the foot gusset. Here's where the crochet hook becomes useful; pick up and knit one stitch in each V on the side of the heel. (A small hole almost invariably appears at this point, where the instep joins the heel flap; to avoid this, pick up an additional stitch in the corner and knit into the back of it, thus twisting the stitch. The hole will disappear as if by magic, and all your sock-knitting friends will be astounded by your scintillating brilliance and unparalleled cleverness.)"