Catalysis of the reaction: 3'-phospho-5'-adenylyl sulfate + UDP-N-acetyl-D-galactosamine 4-sulfate = adenosine 3',5'-diphosphate + H+ + UDP-N-acetyl-D-galactosamine 4,6-disulfate.
uridine diphospho-N-acetylgalactosamine 4-sulfate sulfotransferase activity