Well, just using Poser isn't possible since the polycount is way too high for the Sims. I think you will have to either create the model directly in for instance Milkshape, which is a shareware modeller, or make it in Poser and then reduce the polycount down to a fraction of Poser's native files. I could imagine that it would take a shorter while to build it directly in a low-poly-modeller than to use polyreduction methods, given the amount of postwork this would entail.