A hybrid numerical time domain method is proposed to predict the electromagnetic field around penetrable conductive boxes. The procedure is based on the implementation of the Impedance Network Boundary Conditions (INBCs) in the hybrid Finite-Difference/Whitney-Elements Time-Domain (FD/WE-TD) method. The INBCs model the field propagation inside conductive shield barriers and are used to interface the FDTD and the WETD numerical methods on the surfaces of the shield. The field solution is obtained by the FDTD method in the region outside the box and by the WETD method in the region inside the box. The proposed method is validated in simple configurations.