That is difficult to answer right now.
Basically, The Sims had a very... esoteric content pipeline, which, for objects, required exporting them as accurate bitmaps from 3DS Max in order for them to render properly.
EA Land came with a set of Blender plugins for making objects, but I'm afraid these have been lost in the midst of time.
If anyone can get their hands on them, please let me know!
As for meshes, that shouldn't really be that hard. Since we know how to render meshes, writing a plugin for Blender that can export meshes isn't really that difficult. When it comes to animations, that's more or less the same story.