Hi, there are no mesh smoothing tools in Rhino as one might expect to find in a mesh or sub-d modeler. If you're feeling adventurous, what you might try currently is downloading and installing the t-splines plug-in (beta), converting your mesh into a t-splines object (composed of smooth patches), then a NURBS object, then using the Mesh command in Rhino to re-create the mesh as fine as you need.
You can "smooth" NURBS surfaces, but not in the same fashion as meshes. Remember NURBS are already inherently "smooth" internally, they are not composed of discrete facets as meshes are. If you Mesh-to-NURB in Rhino, you will not get a single surface, but rather one single surface for each mesh facet, which is normally not desirable.
Another way would be to use a free mesh modeler (there are many out there) to do your smoothing. --ch