Add new parameters to [pairtypes] in ffnonbonded.itp

GROMACS version: 5.1.4
GROMACS modification: No
Here post your question: I have a slab geometry with silicon walls, SPC/E water, and potassium chloride. To add the ion-ion and ion-O pairs to CHARMM36 force field, I am planning to apply the parameters from J. Phys. Chem. B 1998, 102, 21, 4193–4204 (
I wonder if I can directly use the position of first peak r_io as sigma and the magnitude of first peak g_io as epsilon in ffnonbonded.itp.
I would greatly appreciate any advice.

[ pairtypes ]
; i    j  func         sigma           epsilon
  O   POT  1            0.28            0.457
  O   CLA  1            0.32            0.406 
  O     O  1            0.275           0.298