Catalyzes the transfer of N-acetylneuraminyl groups onto glycan chains in glycoproteins (PubMed:10742600, PubMed:29251719). Conjugates sialic acid with an alpha-2-6 linkage to N- acetylgalactosamine (GalNAc) glycan chains linked to serine or threonine in glycoproteins. Sialylates alphaGalNAc- and Galbeta1->3GalNAc-O-Ser/Thr epitopes also known as Tn and T antigens. {ECO:0000269|PubMed:10742600, ECO:0000269|PubMed:15466199, ECO:0000269|PubMed:29251719}. This is the function of ST6GALNAC2 (ST6 N-acetylgalactosaminide alpha-2,6-sialyltransferase 2, ENSG00000070731).