how would I make a halfling a monster so I can get the template monster of legend
in the monster manual
Unfortunately not. Lycanthropes don't change type, so the halfling would stay humanoid and not eligible for the monster of legend template. You want to add some template that will turn it into a monstrous humanoid. Tauric (also from MM2) would work, but that might not be the effect you want.Lycanthropy?