Glycosyltransferase which elongates the O-linked glucose attached to EGF-like repeats in the extracellular domain of Notch proteins by catalyzing the addition of xylose. 3-O-(beta-D-glucosyl)-L-seryl-[EGF-like domain protein] + UDP-alpha-D-xylose = 3-O-[alpha-D-xylosyl-(1->3)-beta-D-glucosyl]-L-seryl-[EGF-like domain protein] + H(+) + UDP Belongs to the glycosyltransferase 8 family.