Catalysis of the reaction: [GlcNAc-(1,4)-Mur2Ac(oyl-L-Ala-gamma-D-Glu-L-Lys-D-Ala-D-Ala)](n)-diphosphoundecaprenol + GlcNAc-(1,4)-Mur2Ac(oyl-L-Ala-gamma-D-Glu-L-Lys-D-Ala-D-Ala)-diphosphoundecaprenol = [GlcNAc-(1,4)-Mur2Ac(oyl-L-Ala-gamma-D-Glu-L-Lys-D-Ala-D-Ala)](n+1)-diphosphoundecaprenol + undecaprenyl diphosphate.