Steve, Porter Cable (actually Rockwell back when I bought mine) used to sell the contour pad as an accessory, though I don't see any on their website these days. On the Speedblock model, the normal pad is an aluminum square plate with 1/4"-3/8" felt glued to it, probably pretty similar to your foam pad. It was held to the sander's base by four screws. The contour pad had the same aluminum plate, but with about 3/4" of flexible, fairly soft, closed-cell foam glued to it. To use it, you unscrewed and removed the felt pad and plate and screwed on the contour pad. Since it was thicker, you also needed slightly longer pieces of sandpaper, maybe about 1" longer than the 1/4 sheets that were normally used with the felt plate.
I wouldn't be the least bit surprised if it's possible to get the same effect by just sticking a square of foam (ethafoam packing foam, ensolite foam, etc.)between your existing pad and the sandpaper with no glue and letting the paper itself and the paper clamps on the sander hold it in place. It could always be glued to the standard pad or attached with double-sided tape, but I doubt it's really needed if you snug the paper down tightly over the extra foam chunk. You're not trying to make a drastic change in the way the sander works, you're just giving it a little more cushion so that the pad can conform a bit better to the curved faces of the ribs in the areas with sharp bends.