Here's something that is working for me.
I start out with Posette in a zero pose
import an .obj piece of clothing
move it, size it, etc.....
set parent to body and then save as prop.(smart prop)
open up WW
open up your prop to convert
p4 Posette to p4 Posette
and hit convert.
For this MD apron I used the normall clothing, tight fit and regroup (In WW).
For anything under the hip use a skirt fit or you'll have a slit in the front/back when you move your Posette legs apart.
Have fun!