I was able to add an 2.5 Samsung Evo 850 SSD with this adapter from OWC.
http://eshop.macsales.com/shop/SSD/PCIe/OWC/Mercury-Accelsior/S-Carrier
Works fine.
I also added an Nvidia GT 610 graphics card.
and PCI Sound Blaster Audigy 2 zs Sound Card with Live Drive
Apple Broadcom WiFi-ac PCIe
IOgear Bluetooth USB adapter
All work vanilla in Windows-8 or 10 ( Plug and play )