Here's one that will allow you to use it in a card slot of an older Macpro.
http://www.ebay.com/itm/2013-2014-M...-4X-adapter-/232076338162?hash=item3608d3e3f2
I did some searching, and there doesn't seem to be any solution for what you want to do.
I think that part of the reason is that the PCIe SSDs can be seriously faster than the SATA bus.
Perhaps there are timing issues with a PCIe storage device operating through an SATA bus.
The PCIe to USB 3.0 enclosure from OWC actually is a decent solution, as it is fairly small, and going through a USB 3.0 connection, will be nearly as fast as using one installed internally through SATA.