I guess you have looked around in this subforum, and found that (AFAIK) nobody succeeded in getting Lacie firmware running on empty disks? It is certainly possible (Lacie can do it), but you'll need in-depth knowledge about how Linux boots, the possibility to read and interpret scripts, and a serial port on your nas. So far there is no tutorial.
At the moment an alternate firmware for the 5big is being developed. You could install that, and volunteer for testing the raid support (which is not added yet). http://plugout.net/