There are "booster" power supplys that take up a 5.25" drive bay or two that will power a GPU. Its usallycheaper/easier to just replace the PSU. They often sell for $40+, at which price your better off with the Corsair 400W.
I agree - though the GPU PSU's do have some uses - for example slimline cases with non ATX size powersupplies that do not readily have powerful replacement models available - and for those that do not know how to install a PSU they make things easier.
If you can't change a PSU, what makes you think they can add that?
Forgot to mention the problem with cooling/noise. Those things normally use two 40mm fans, and they get LOUD. You are REALLY better off using one good PSU. Good point on the slim line cases, but those to usually only have one 5.25 drive, and that normally has a DVD drive. There could be cases where this is a good idea, but normally not.