I'm pretty sure that once you've got an accessory resized to fit a Sim, the hard part from there is animating-- linking is fiddly, but if you leave each age that needs a mesh as a separate package there's a tool to link them automatically (just bundle the ages afterwards, the game can't tell). I was just talking about it with Nixnivis over on her Wordpress.

And I'll be honest, those look good enough that my main concern is the polycount. I know Milkshape's tools for reducing poly count are pretty terrible, but how high poly are we talking here? The wireframe hat looks practically solid.

(Still no steeple hennins anywhere? It's like the internet conspires against me.)