Two ways: 1) Use a potion (easiest way). Essence of Lycan, or something like that. Either check the store until it sells one or make one yourself. The recipe isn't that hard. Requires silver, wolfsbane and something else, if I recall correctly. 2) Get a werewolf to use a cursed bite on you. I say this is harder because I don't like having to befriend other Sims to accomplish things, and I think to get a werewolf to bite you you probably need to be friends with them first. Also, I think you have to UNLOCK the cursed bite ability, so not all werewolves may be able to do this (although I could be mistaken).
Why do you need to 'unlock' it? Cursed Bite is a normal social interaction for werewolves. You have to befriend the Sim first, but on full moons you can use it successfully on Sims. I know because I did it myself and the Sim I never met before turned into a were.
Max: "I think he just needs a hug, or a sharp blow to the head." FC 4468-1025-6646
If you have enough lifetime happiness saved up, buy the "Alpha Wolf" reward. It should allow you to give any human Sim the Cursed Bite regardless of relationship status or time of day. Note: you will, of course, need to be in your wolf form to do so.