Lithium nitride has recently emerged as a promising material for hydrogen storage. The hydrogen storage capacity reaches 10.2 wt% H by the formation of compounds, such as imides, amides, and others. Hydrogenation of lithium nitride is highly exothermic, and thus desorbing hydrogen from these compounds requires high temperature and cannot be used for reversible hydrogen storage. Ab initio calculations predict that partial substitution of Li by transition metals like Cu or Ni can reduce the reaction enthalpy between amide and imide. In this work, we present the synthesis of the ternary system Li3−xMxN (M = Co or Ni) by Spark Plasma Sintering (SPS). The samples are hydrogenated at 255 °C by solid gas reaction. The sample crystal structures have been analyzed by synchrotron X-ray powder diffraction using a high resolution powder diffractometer. The structural models for Co and Ni-substituted Li3N have been confirmed The effect of the substitution on the phase formation upon hydrogenation has been investigated at various metal and hydrogen concentration. Different behaviors are observed depending on the nature of M.