Is it possible to incorporate the empty d-orbitals of a p-block element (suppose it is chlorine) into a VASP calculation? If yes, what is the standard procedure to incorporate these orbitals.
If I understand the question, you want to add Hubbard U correction to d orbitals on chlorine atoms? I think you can get bands with d-orbital character by including enough empty bands, but I don't think there is any point. The Hubbard U correction term only adds an energy penalty between occupied orbitals and the d-orbitals will not be occupied (unless you add lots of electrons). An analogous argument is true with e.g. the Hartree potential; the unoccupied states don't contribute to the density and don't alter the potential.
The empty orbitals are always included in plane wave calculations. The plane waves don't conform to atomic symmetry, and can be seen to contain contributions of arbitrary angular momentum.
If you were to use an atomic basis set, then you would need to put in D basis functions explicitly; however, this is pretty much always done in practice at least in the quantum chemistry community, since you need D fuctions to describe polarization of the P orbitals. Typically, a polarized double-zeta basis will have D functions for Cl, a triple-zeta basis will also have F functions, and a quadruple-zeta basis will have G functions.