OR...... (Do NOT do this - its too simple and therefore dangerous!) - you could put a THICK pad of plywood UNDER the floor jack to increase its overall height. Make sure U use hardwood plywood so wheels don't cause dents which, in turn, cause instability. Maybe some ply with a sheet of steel on top of it?
The more important Q; What the heck are you doing that requires you lift it so high? Please say U R not going under there while its on a jack!