Do you mean you want to zoom in on a specific point instead of the center? In that case you need to move the point of interest by holding down "Shift+Alt" and then clicking on the subd surface where you would like to zoom towards.
Or do you mean you want to scale the object around a point other than the center? In that case you need to use the "Set Pivot" tool and move the pivot to the point you want to use instead of the center. (you might also be interested in the Non-proportional scale tool if you only want to scale in a single axis)?
