Catalysis of the reaction: N-acetyl-D-galactosaminyl-(1,3)-D-galactosyl-(1,4)-D-galactosyl-(1,4)-D-glucosylceramide + UDP-N-acetylgalactosamine = N-acetyl-D-galactosaminyl-N-acetyl-D-galactosaminyl-(1,3)-D-galactosyl-(1,4)-D-galactosyl-(1,4)-D-glucosylceramide + UDP.
UDP-N-acetyl-D-galactosamine:N-acetyl-D-galactosaminyl-1,3-D-galactosyl-1,4-D-galactosyl-1,4-D-glucosylceramide alpha-N-acetyl-D-galactosaminyltransferase activity