Oki

You open DR Pixels' mesh with Simpe
From the 4 files you select the gmdc
Right click on it and extract the file(save it)
This extracted file you can open with Milkshape.(you import it with the Unimesh plugin from Wes H)
You can edit it, or import a set of shoes
When done you export it with Unimesh
Then open the mesh package again with Simpe and replace the gmdc(right click on it)with your saved file)
Thats all
Note
You can never upload your new mesh(share it) because it is still DR Pixels mesh.
If you want to make a new one(wich is smarter)then try this tutorial:
http://personal-computer-tutor.com/sims2/suzysclothingtut.htm