How to create a .nxs file??

How to create a .nxs file??
And also, Can I convent Ogre::Mesh to NxOgre::Mesh ??

I've used Flour to do it from a .mesh file.

With the last version of Flour (0.4) e should first convert your mesh to a ".flower" file (using a exporter from your 3D Software, like Blender) then with that flower file you use Flour to convert it to a ".nxs" file.

Good Luck