I hit my volleys on both sides with one hand and with a continental grip. There is, however, a way to hit a forehand volley with topspin but it is unorthodox. It involves using a two-hand forehand. If you're right-handed, the left hand goes above the right-hand. At the point of impact with the ball, roll your wrists slightly forward (left supinating, right pronating). You'll get some small degree of topspin on the volley, but it's only effective if you're angling it wide to the left, i.e., your opponent's right side. All in all, it's probably not worth the trouble to learn.
Salt air, red wine, olive oil, & gaudium certaminis (Latin for "joy of battle").