The effects of conduction-band (CB) impurities in heavy-fermion systems are studied on the basis of the periodic Anderson model in the framework of a slave-boson mean-field theory within the single-site coherent-potential approximation (CPA). We provide a simplified CPA formalism for the treatment of mixing disorder induced by CB alloying. The density of states for conduction and localized f electrons can be calculated self-consistently in the whole impurity-concentration range. The concentration dependence of the specific-heat coefficient, static magnetic susceptibility and resistivity at low temperatures as well as the Kondo temperature in the alloys is obtained. The results indicate that the experimental observations in upon doping with Au, Ag and Al can be qualitatively explained as the effects of mixing disorder.