I recently added a gusset to a sweater in order to enlarge it. It had been knitted in two pieces, from cuff to center (at right angles to the usual way of knitting) and had kimono style 3/4 sleeves. I added the gusset by knitting up stitches along the lateral selvedge of each finished piece, that is: up from cuff edge along the underside of the sleeve, underarm, and and then down the side to the bottom edge. The only mistake I made was that, in fact, I didn't need it! The sweater is now too big for me.