*blurg*
the first handicap of this mesh is that it doesn't have layers that you might exploit for this purpose. It is done all as one piece, but very well mapped so that is a stroke in its favour.
I had never actually looked at the mesh in SimPE until today, and it is hideously high poly

over 10 000 faces.
Milla (clubkitty) made it a habit to pirate Poser meshes and post them for The Sims 2, the Jessica Rabbit style hair was the final act of Poser-mesh-piracy that ended her career and she withdrew from the community under a cloud of shame... those obviously were different days -- but we can't ask her where she found the orignal so I can ask for permission to reduce the polycount and make this mesh with alpha layers (which is the thing we might want to do).
So instead we must approach this from a different angle. You will need to do a lot of experimenting, but I have extracted a UVmap that may make this easier. Because the mesh is so high in poly-count, I had to go very large with the UVmap image so the details of the mapping wouldn't be lost (you will find it in the ZIP at the end of the post - the JPG is just so you can see why I made the GIF so large) - but you would need to increase the size of the image you want to edit to 2048x2048 to make the parts line up.
It will be a mind-hurting thing at first, but once you figure out the locations of the zones you want to add the details, then you can make a transparent PNG with the details. The idea here is to have the cloth as the bottom layer in the image editor, the UVmap as a layer at the top of the stacks, then in the middle a transparent layer that you make your details like piping, edging, and the like.
Then delete the UVmap layer and the bottom cloth layer - save as a transparent GIF or PNG (I would use PNG-24)
You would resize the new PNG to 512x512.
then open the cloth textures and import the new PNG as a layer or copy and paste as a new layer, use the colour adustment tools to make the details suitable to your taste, then flatten the layers and save as a BMP for use in Body Shop.
I hope this makes sense. I am never very good explaining Gimp, I have spent too many years in Photoshop =(