Split polygon tool obviously only works on polygons.

The reason it's not bulging, I imagine, is because there's not enough geometry. For nurbs, you can use the Insert Isoparm command.

In Isoparm component mode, click and drag an isoparm to where you want to add a new one. This won't move the isoparm, but it will createa dotted line at the new location. With the dotted line in place, use the Insert Isoparm command from the Edit Nurbs menu.