A nurbs object is only mathematically capable of having 4 sides (although fairly unstable 3-sided nurbs objects are also possible), therefore, a true Nurbs cube is only possible (that I've found anyway) by taking a Nurbs sphere and manipulating it to be cube-ish in shape (ie. see image).

Add more isoparms at the edgs to get sharper geometry.

No, polygons have different setups then Nurbs when it comes to texture coordinates (UVs). The positive thing, though, is you can manipulate a polygon's UVs however you want, while a nurbs' object's UVs are pretty much stuck where they are.