Doping and substitution methods are predominantly employed in the synthesis of ceramics to achieve their desired functional properties. We studied the behavior of excessive dopants in addition to an existing stoichiometric composition using a high-throughput continuous compositional spread sputtering method. We paid attention to the possible formation of thermodynamically unstable phases by the addition of an excessive amount of dopants. We showed that even when dopants were added as an additive, they dissolved into the existing lattice due to the benefit of the entropy of mixing. Regardless of excessiveness, all added elements incorporated into the lattice, stabilized by the tolerance factor. We also demonstrated our findings exemplarily with lead iron niobate to induce magnetic properties alongside inherent ferroelectricity (M_S = 10 emu/cm³, P_S = 16 μC/cm²). We compare the results from CCS with those from the non-additive solid-state method, leading to a conclusion that the benefit from the entropy of mixing allows foreign elements to substitute for the elements initially residing in the lattice to a degree in compliance with the Goldschmidt tolerance factor. This observation was confirmed by a density functional theory calculation. We anticipate that our study could necessitate intensive research on achieving desired composition through industry-friendly processing.