Sequential adsorption of N atoms on a MgO(100) supported Fe-7 cluster is studied using a density functional approach. For the number of adsorbates varying between one and six, the most favorable adsorption geometries are determined and the corresponding potential energy diagram is given. Up to five N atoms are found to bind strongly to the oxide supported cluster, with binding energies ranging from 0.98 to 1.11 eV per N atom. When a sixth N atom is added to the Fe-7/MgO(100) cluster with five preadsorbed N atoms, the potential energy sharply increases due to the strong repulsive interaction between N adsorbates at short distances. The MgO(100) support plays an important role in increasing the binding energy of the adsorbed species.