Adds the first GalNAc residue on to the isoprenoid-linked bacillosamine (2,4-diacetamido-2,4,6-trideoxyglucose) carrier in the N-linked protein glycosylation pathway. Acts first on the undecaprenylpyrophosphate-linked bacillosamine (Und-PP-Bac) substrate to yield the disaccharide. N,N'-diacetyl-alpha-D-bacillosaminyl-tri-trans,hepta-cis-undecaprenyl diphosphate + UDP-N-acetyl-alpha-D-galactosamine = H(+) + N-acetyl-alpha-D-galactosaminyl-(1->3)-N,N'-diacetyl-alpha-D-bacillosaminyl-tri-trans,hepta-cis-undecaprenyl diphosphate + UDP Protein modification; protein glycosylation. N-linked protein glycosylation in C.jejuni consists in the transfer of a heptasaccharide (GalNAc-alpha1,4-GalNAc-alpha1,4-(Glcbeta1,3)-GalNAc-alpha1,4-GalNAc-alpha1,4-GalNAc-alpha1,3-bacillosamine) from a membrane-anchored undecaprenylpyrophosphate (Und-PP)-linked donor to the Asn side chain of proteins at the Asn-X-Ser/Thr motif. Belongs to the glycosyltransferase group 1 family.