Catalyzes the addition of N-acetylglucosamine (GlcNAc) in beta 1-6 linkage to the alpha-linked mannose of biantennary N-linked oligosaccharides. N(4)-{beta-D-GlcNAc-(1->2)-[beta-D-GlcNAc-(1->4)]-alpha-D-Man-(1->3)-[beta-D-GlcNAc-(1->2)-alpha-D-Man-(1->6)]-beta-D-Man-(1->4)-beta-D-GlcNAcl-(1->4)-beta-D-GlcNAc}-L-asparaginyl-[protein] + UDP-N-acetyl-alpha-D-glucosamine = H(+) + N(4)-{beta-D-GlcNAc-(1->2)-[beta-D-GlcNAc-(1->4)]-alpha-D-Man-(1->3)-[beta-D-GlcNAc-(1->2)-[beta-D-GlcNAc-(1->6)]-alpha-D-Man-(1->6)]-beta-D-Man-(1->4)-beta-D-GlcNAcl-(1->4)-beta-D-GlcNAc}-L-asparaginyl-[protein] + UDP Protein modification; protein glycosylation. Expressed in a complex subset of neurons in larvae and in the spermathecal and pharyngeal-intestinal valves and certain vulval cells of adults. In embryos, expressed from the late comma stage. Belongs to the glycosyltransferase 18 family.