Catalyzes the transfer of a glucose moiety from UDP-glucose to another glucose that is N-linked to an asparagine within a peptide or protein. Can act in a repetitive manner, and this way it elongates the N-linked glucose by a glycan chain consisting of several alpha-1->6 linked glucose residues. Is able to add up to six glucose units in vitro. Cannot use UDP-Gal, UDP-GlcNAc or UDP-GalNAc as a substrate donor. Does not require a metal cofactor. Protein modification; protein glycosylation. Belongs to the glycosyltransferase group 1 family.