Catalysis of the reaction: alpha-N-acetylneuraminyl-(2->3)-beta-D-galactosyl-(1->3)-N-acetyl-D-galactosaminyl-R + CMP-N-acetyl-beta-neuraminate = alpha-N-acetylneuraminyl-(2->3)-beta-D-galactosyl-(1->3)--N-acetyl-D-galactosaminyl-R + CMP.
(alpha-N-acetylneuraminyl-2,3-beta-galactosyl-1,3)-N-acetylgalactosaminide alpha-2,6-sialyltransferase activity