In this work, we propose an extension of the Pitzer-Debye-Huckel (PDH) model for solutions containing ionic species at all concentrations. In the original PDH model, the ionic species are excluded from the solvent, so their dielectric response due to the presence of other ions is ignored. This may not be appropriate at high concentrations of the ionic species. We resolve this problem by starting from the excess Gibbs free energy of the PDH model and including the contribution to solvent-related properties (i.e., molecule weight, dielectric constant, density, closest approach parameter, and Debye-Huckel parameter) from all species in the solution. We show that the activity coefficient of both ionic and neutral species derived from such a model not only satisfies the Gibbs-Duhem relation but also improves the description of ionic liquid solutions over the whole concentration range when combined with the predictive COSMO-SAC activity coefficient model for short-range interactions. When compared to the original PDH model (which considers only the neutral species as a solvent), the new method (inclusion of IL as a cosolvent) improves the prediction of IDAC by 46% (3555 data points), VLE by 12% (1682 data points), the osmotic coefficient by 22% (1372 data points), and LLE by 33% in the IL-rich region and show comparable results in the solvent-rich region of LLE (2876 data points).