Protein glycosylation is the most complex post-translational modification process. More than 50 % of human cells proteins are glycosylated, whereas bacteria such as E. coli do not have this modification machinery. Indeed, the carbohydrate residues in natural proteins affect their folding, immunogenicity, and stability toward proteases, besides controlling biological properties and activities. It is therefore important to introduce such structural modification in bioengineered proteins lacking the presence of carbohydrate residues. This is not trivial as it requires reagents and conditions compatible with the protein's stability and reactivity. This work reports on the introduction of lactose moieties in two natural proteins, namely ubiquitin (Ub) and l-asparaginase II (ANSII). The synthetic route employed is based on the sulfur(VI) fluoride exchange (SuFEx) coupling of a lactose tethered arylfluorosulfate (Lact-Ar-OSO2 F) with the ϵ-NH2 group of lysine residues of the proteins. This metal-free click SuFEx reaction relies on the properties of the fluorosulfate employed, which is easily prepared in multigram scale from available precursors and reacts chemoselectively with the ϵ-NH2 group of lysine residues under mild conditions. Thus, iterative couplings of Lact-Ar-OSO2 F to Ub and ANSII, afforded multiple glycosylations of these proteins so that up to three and four Lact-Ar-OSO2 groups were introduced in Ub and ANSII, respectively, via the formation of a sulfamoyl (OSO2 -NH) linkage.