Off the top of my head I don't think any consumer NASes support add in cards. Generally the off the shelf NASes only allow additional hard drives, usb/esata drives, and maybe a printer. Some also allow for additional memory to be added. I would say you would have to build your own NAS if you plan on using a particular PCIe card. I could be wrong on that though so I would wait for others to respond to get a better idea if it is possible or not.
00Roush