The calculation of electromagnetic (EM) fields and waves inside finite-sized structures comprised of different media can benefit from a diakoptics method such as linear embedding via Green's operators (LEGO). Unlike scattering problems, the excitation of EM waves within the bulk dielectric requires introducing sources inside the structure itself. To handle such occurrence, we have expanded the set of LEGO sub-domains - employed to formulate an EM problem - to deal with the inclusion of elementary sources. The corresponding subdomains (bricks) play the role of ``generators'' in the equivalent model. Moreover, if a source is ``turned off'', as it were, the enclosing brick can be utilized as a numerical ``probe'' to sample the EM field. In this paper, we present the integral equations of LEGO modified so as to accommodate generator/probe bricks. Numerical results are provided which demonstrate the validity and the efficiency of the approach.