How to set Mixture Atom using Dummy Atom?

Hi All.

I'm thinking of expressing the function of "atoms occupancy" as a "mixture atom".
Because I want to run "Random Alloy Stability and Properties(Cluster Expansion)" Protocol which input file is electrode(ex. LixNiO2(x=0.0, 0.2, 0.5, .., 1.0)).
How to set Mixture Atom using Dummy Atom??