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 condensation of 2,4-diacetamido-2,4,6-trideoxymannose with phosphoenolpyruvate (PEP) to give N,N'-diacetyllegionaminic acid. 2,4-diacetamido-2,4,6-trideoxy-alpha-D-mannopyranose + H2O + phosphoenolpyruvate = N,N-diacetyllegionaminate + phosphate