Morphotropic phase boundary (Bi1/2Na1/2)TiO3(Bi1/2K1/2)TiO3 (BNTBKT), was modified with increasing additions of Bi(Zn1/2Ti1/2)O3 (BZT). Microstructure, electric-field-induced strain and polarization, dielectric permittivity, and temperature-dependent piezoelectric coefficient were investigated and compared with crystal structure measured in situ as a function of applied electric field. Furthermore, permittivity and piezoelectric coefficient were characterized as a function of electric field. For small additions of BZT, an applied electric field leads to an irreversible phase transition into a ferroelectric phase with remanent polarization and a reduced relative permittivity. Increasing the content of BZT increased the threshold field for the transition. For additions of more than 2 mol% BZT, the piezoelectric coefficient dropped, permittivity remained almost constant, and a high normalized strain of up to 500 pm/V was observed. However, no field-dependent structural change was evidenced by the in situ X-ray experiment.