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-un.