Not in the same manner as you can with polys, but you can simulate the effect through a bit of work. First, select the surface patch to be extruded. Then duplicte that patch (under the edit NURBS menu). With the newly create surface, switch to Isoparm mode and select the Isoparms that border the new surface. Extrude these isoparms. You have new surface in the direction of the extrusion, but you have to cap them off. Select one of extruded side surface, switch to isoparm mode, and select the end isoparm where the cap will go. Shift select another side surface and repeat this until you have the end Isoparms selected. With them all selectc, select surfaces-->boundry. This should create another surface which caps the side surfaces.