We show that double-resonance spectra recorded during the simultaneous absorption of x-ray and microwave (MW) photons are a fingerprint of the perturbed electronic configuration of atomic species driven to ferromagnetic resonance. X-ray absorption measurements performed as a function of x-ray energy and polarization over the Fe L2,3 edges of single-crystal yttrium-iron garnet reveal MW-induced multiplet features related to angular momentum transfer from the MW field to localized Fe 3d magnetic sublevels. O K-edge absorption spectra demonstrate the formation of dynamic 2p-orbital magnetization components at O sites coupled to the Fe magnetic moments at tetrahedral and octahedral sites. These results are compared with double-resonance x-ray absorption spectra of Permalloy, showing that the MW transition probability is distributed according to the hybridization character of the 3d states and proportional to the unperturbed unoccupied magnetic density of states of metals and insulators.