a 3-O-[beta-D-galactosyl-(1->3)-N-acetyl-alpha-D-galactosaminyl]-L-threonyl-[protein] + CMP-N-acetyl-beta-neuraminate => a 3-O-{beta-D-galactosyl-(1->3)-[N-acetyl-alpha-neuraminosyl-(2->6)]-N-acetyl-alpha-D-galactosaminyl}-L-threonyl-[protein] + CMP + H(+)