Hi Lejan,
You can look at this website here:

http://www.resurf3d.com/

They have a couple standalone applications that do what you are describing.. The mesh to single nurbs one will fit to an "open mesh" (A sheet if you will) not a closed volume. it works off of stl's.

The obj mesh to solid, will work on closed volumes, but is only approxamating what it works on...

The expensive ones michael mentions will do much better jobs, but again, EXPENSIVE.

Also, you can try these demos for a few days... What you will find, is a nurbs surface fit to some organic shape, at any type of tolerance, will be an Extremly heavy surface created... And then, "What do you want to do with it in MoI?" A very heavy, organically shaped NURBS surface may become "unusable" in any type of situation...

You can check those out to see results, but the higher end ones may really be needed to presnt anything that suits what you want.. Maybe the heavy stuff will be good for what your looking for though...
Burr