No, a mesh is composed of multiple flat faces that don't blend into faces it shares edges with. A nurb surface is a single continuously curved and blended surface. Two nurb surfaces or more can enclose a watertight volume, and it's regarded as a solid if the CAD app treats it as if it's 'filled', ie slice or split it into two and separate the pieces, the split plane or surface produces surfaces that seal the cavities inside them.
Tim demonstrated some mesh to nurb conversion here: http://forum.punchcad.com/showthread.php?t=4428 and gives some reason to hope that this type of capability might show up in PunchCAD, but I'd expect it to be a premium feature that would probably be exclusive to Shark. If you can find it, having an automatic capability like this comes at a pretty high price right now.