Hiya,
there is few plugins you can find on this site to create shadow (can rember the names) i think there is also some hidden fonctions inside nuke, the foundry not really showed because it was buggy from an old version.
otherwise you will have to wait that nuke get a proper way to do it. There is already a particul system i saw running 2 years ago and now the deal with sony picture Katana.... it could come at some point.
in the meantime, nuke is not a 3D software, it is a compositing one. Even if i also will appreciate to be able casting shadows somtimes. A complexe setup that require a lot of shadows especially raytraced, would be better done on maya or an other 3D package.
You can export you cam from nuke to any soft.
cheers