an N-acetyl-alpha-neuraminyl-(2->3)-beta-D-galactosyl-(1->3)-N-acetyl-beta-D-galactosaminyl derivative + CMP-N-acetyl-beta-neuraminate = an alpha-Neu5Ac-(2->3)-beta-D-Gal-(1->3)-[alpha-Neu5Ac-(2->6)]-beta-D-GalNAc derivative + CMP + H(+) — reaction