Catalysis of the reaction: N,N'-diacetyl-alpha-D-bacillosaminyl-diphospho-tri-trans,hepta-cis-undecaprenol + UDP-N-acetyl-D-galactosamine = N-acetyl-D-galactosaminyl-alpha-(1->3)-N,N'-diacetyl-alpha-D-bacillosaminyl-diphospho-tri-trans,hepta-cis-undecaprenol + UDP + H+.
N,N'-diacetylbacillosaminyl-diphospho-undecaprenol alpha-1,3-N-acetylgalactosaminyltransferase activity