3-O-[alpha-Neu5Ac-(2->3)-beta-D-Gal-(1->3)-alpha-D-GalNAc]-L-Ser-[protein] + CMP-N-acetyl-beta-neuraminate = a 3-O-{alpha-Neu5Ac-(2->3)-beta-D-Gal-(1->3)-[alpha-Neu5Ac-(2->6)]-alpha-D-GalNAc}-L-seryl-[protein] + CMP + H(+) — reaction