Catalysis of the reaction: 1,2-diacyl-3-beta-D-galactosyl-sn-glycerol + UDP-D-galactose = 3--1,2-diacyl-sn-glycerol + H+ + UDP.
UDP-galactose:3-(beta-D-galactosyl)-1,2-diacyl-sn-glycerol 6-alpha-galactosyltransferase activity