I have an object which is subdivision surface to nurbs and although as a subdivision surface is ONE object, after converting it to nurbs it is divided in many nurbs surfaces..the problem is that I want it to be ONE object again and stiching or attaching the nurbs surfaces doesn't work properly..do you have any idea how I can have one nurbs object again??
Unfortunately NURBS don't work the way you want them to. They cannot have arbitrary shapes like polys and subds can. NURBS are always, at a fundamental level, squares. The only way to get holes or branches or extrusions using them is to arrange several of them together very carefully.
Think of NURBS like pieces of cloth. You can't build a shirt out of just one piece of cloth; you need to cut and stitch several pieces together because shirts have complicated shapes, and cloth is basically square. It's the same with NURBS. Read your docs for more information.