a 3-O-[N-acetyl-alpha-neuraminyl-(2->3)-beta-D-galactosyl-(1->3)-N-acetyl-alpha-D-galactosaminyl]-L-threonyl-[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-threonyl-[protein] + CMP + H(+) — reaction