Catalysis of the reaction: (1->6)-alpha-D-mannosyloligosaccharide + beta-D-mannosylphosphodecaprenol = (1->6)-alpha-D-mannosyl-(1->6)-alpha-D-mannosyl-oligosaccharide + decaprenol phosphate.
beta-D-mannosylphosphodecaprenol:1,6-alpha-D-mannosyloligosaccharide 1,6-alpha-D-mannosyltransferase activity