I have got the polygon representation of one NURBS surface by gl feedback mechanism. Could I convert these polygons into triangles or directly convert NURBS surface into triangles?

I believe that such polygons will be convex, so you can split them easily in triangles.

i.e : polygon with vertices 0,1,2,3,4 (pentagon)

-> triangle 1 : 0,1,2

-> triangle 2 : 0,2,3

-> triangle 3 : 0,3,4

and you are done.