The KIKO3DMG is a nodal reactor-physics calculation tool. This paper presents extensions, which were necessary to prepare the code for simulation of fast reactor transients. A new axial expansion module was developed for the KIKO3DMG, which introduces mixed nodes. The theoretical background and a new method of group constant mixing are described. Weighting the group constants with the volume ratios is appropriate for small, non-absorber nodes, if the inverses of the diffusion coefficients are weighted instead of the diffusion coefficients. Group constants were generated for the ALLEGRO helium-cooled fast reactor using the Serpent Monte Carlo code and applied in the KIKO3DMG. The effect of the group constant parameterization was investigated on the power distribution and the coefficients of reactivity. The axial and radial power distribution and the fuel, cladding and wrapper temperature coefficients of reactivity and the void coefficient of reactivity were calculated with good accuracy in the KIKO3DMG.