If you don't mind hacks you can use the Insiminator. I think that I just found it by googling it when I heard other people refer to it. It's a very useful hack, but use carefully because it can make your game buggy. I think I may have just had another hack on there that it didn't like but I had to use the Custom Content Manager to turn it off. Now I just turn it on long enough to change something in the game, usually if I want one of my sims to have a specific career or to have a kid with another sim but I don't want to wreck a family by making the husband or wife have an affair.
I believe Merola's Painting will also do the same thing and is probably easier to use than the inSIMinator...(I wouldn't be without it!)
