Ok I can respect you being cautious, too many people on here have added drives in the wrong way to their pools and then get stuck up a creek without a paddle.
Good news there is no way you will lose your data, no matter how you add those drives to your FN box. The only caution you have to look out for is to add the 8 new drives to your existing pool with the correct party (raidz1, raidz2, radiz3, etc.). Many people make mistakes when they try and add 1 or 2 drives or some low number of drives, and then accidentally stripe it as a single disk with no parity. The danger is that once you add a vdev to a pool, there is
no way to remove it, without destroying the whole pool (the new vdev and all other previous vdevs) and rebuilding.
If you plug all 8 drives in and add them at the same time (which you need to do, you obviously can't add drives one by one) then you will avoid the "added a single drive to my existing pool, oh crap". Just make sure it says "raidz2" when you are adding them and you're good to go. If you happen to mess up and add the 8 new drives as a new poo instead of a new vdev, no biggie (this would happen if you don't select your existing pool when you add the new drives. if you don't do that, it assumes you want to add a new pool). You can destroy the new accidental 8 drive pool and then add it again properly as a vdev. no biggie.
I hope I made this more clear, not muddier. Ask more questions if you have them. Best practice is to follow the manual procedure for this:
http://doc.freenas.org/9.10/storage.html#volumes