How can I add edge loop in nurbs?

Try to use the Carve SOP and Merge SOP for combining with the original geometry.

No real way to add edge loops on NURBs patches in Houdini.

You can use the Refine SOP to add isoparms exactly where you want them. To create a “loop”, you can add four isoparms by enabling the two u and two v toggles. You can use the handles in the viewport to put the isoparms where you want.