A method to integrate metasurfaces in lens design is proposed. Thanks to a model based on effective medium theory used in conjunction with lens design software, the optical properties of custom metasurfaces within a complex system can be calculated. Compared to simple phase surfaces embedded in common lens design software, the phase shift profile given by our model is dependent on the wavelength and field of view. We thus demonstrate that those phase surfaces cannot accurately reproduce the behaviour of metasurfaces in every system. As a proof- of-concept, a hybrid refractive and metalens design was also made. The metasurface located within two refractive lenses corrects mainly the spherical aberration and allows the system to be almost diffraction-limited for the whole field of view.