They sell these at my local electronics store, Fry's. These are really made by Metra, like all the other adapter plates. That's a little expensive, considering they're $15 right at the normal store.
But yes, they are so you can fit LARGER speakers, the 6x9s into a 5x7 hole. I don't think there would be a reason anyone would want to put a smaller speaker in a larger hole.
I took me a long time to try and read the instructs on through the plastic package, but it wouldn't work unless you got a really shallow 6x9. You're suppose to mount the adapter behind the 5x7 mount, so you lose at least an inch of depth, and then mount the 6x9 behind it. If you tried to do it the other way, the speaker would stick out an inch, and I don't think the 6x9 basket would fit since the adapter immediately starts to shrink to 5x7.
The only place these might work is in the rear area of the Sports.