Involved in biosynthesis of legionaminic acid (5,7-diamino-3,5,7,9-tetradeoxy-D-glycero-D-galacto-non-2-ulosonic acid)(Leg), a sialic acid-like derivative that is incorporated into flagellin via O-linkage to Ser/Thr. Catalyzes the conversion of N,N'-diacetyllegionaminic acid (Leg5Ac7Ac) and CTP into CMP-N,N'-diacetyllegionaminic acid (CMP-Leg5Ac7Ac). CTP + N,N-diacetyllegionaminate = CMP-N,N-diacetyllegionaminate + diphosphate Belongs to the CMP-NeuNAc synthase family.