you might be better off picking up a Mcgivern gold bead front sight and replacing the existing sight rather than attempting a modification. (assuming you are working on a smith)

If you are still interested in gluing a bead on an existing sight blade I might consider a cyanoacrylate adhesive, not sure how it will stand up to recoil and abuse or cleaning solvents. A more permanent adhesive would be to use a two part epoxy but you will likely damage the partridge front sight when you try removing the bead.