We show a method to construct model Hamiltonians for describing multiplet excitations of transition-metal impurities. Here we treat systems of a transition metal substituting Al in α-Al2O3. Based on the results of quasiparticle self-consistent GW (QSGW) calculations for the systems, we construct the model Hamiltonian of 3d orbitals. We determine not only the crystal field parameters but also the parameters of effective interactions based on the theoretical correspondence between QSGW calculations and the model Hamiltonian, and investigate systematic change of these parameters. Finally, we discuss the structures of multiplet excitations calculated from these parameters.