3-O-[(D-ribitylphospho)2-3-N-acetyl-beta-D-galactosaminyl-(1->3)-N-acetyl-beta-D-glucosaminyl-(1->4)-O-6-phospho-alpha-D-mannosyl]-L-seryl-[protein] + UDP-alpha-D-xylose = 3-O-[beta-D-xylosyl-(1->4)-(D-ribitylphospho)2-3-N-acetyl-beta-D-galactosaminyl-(1->3)-N-acetyl-beta-D-glucosaminyl-(1->4)-O-6-phospho-alpha-D-mannosyl]-L-seryl-[protein] + UDP + H(+) — reaction