An Adapter!?!?! Im sorry dude, but it just. doesn't.work.like.that.
Even if you where to somehow able to replace the slot (impossible) its not like the motherboard is gonna have the hardware to support it. Its the equivalent of cutting the socket of an American appliance an replacing it with a European one. Sure it will fit, but the results will be less than pleasant.
. Im sure you have a reason for not doing so, but why don't you just put (insert pci-eX16 card here) into the computer with the x16 slot? or just switch the mobos?