There's really only one choice: Milkshape 3D. Once The Sims really took off, the programmers included native .SKN translators. (Somebody wrote a Sims2 plugin for it, too.)
The catch is, it is a pay program and rather pricey ($35 US). You do get a 30 day free trial to see if it suits your needs, but after that - pay up or forget it.
Link:
http://www.milkshape3d.com/