I am not sure about where your equations come from, but my guess is that this relation holds for a continuous material only (i.e., zones in 3DEC).
In 3DEC, kn and ks relate to the joints (i.e., contacts), while the Poisson ratio relates to the rock matrix (i.e., zones) and there is absolutely no relation between those.
Since you consider a rigid body, setting a Poisson ratio to the zones will not affect your model. Still, rigid bodies will interact through the joints, and those need at least the kn and ks.