maybe a little more information from you would be helpful in determining the best way forward.
- how much raw data do you have?
- do you have some place this can be stored off the current pool?
- do you have proper backups of your data that is on FreeNAS?
- at your current rate of data growth, how much raw storage should you add so you don't have this same problem in 6 months.
- what is your budget?
if you want to expand your current raidz2, you can not without destroying it and starting over. this would require you to temporarily move the data off the pool then reconfigure, load it back on.
so you have two options:
1) expand the current radiz2 you have by however many more disks (maybe 2) you want to add (i believe it is not recommended to go beyond 12 disks in raidz2 but that is even stretching it). this would require you to offload the data to some external drives, add the disks, setup a new pool with raidz2 x however many disks you want, then load the data back on. the number of disks you add depends on your budget and your needs into the future. this will depend on your budget and how many disks you can purchase right now
2) probably the slightly better option if you can afford it is to just create another raidz2 pool with 6 newly purchased disks with the greatest capacity you can afford. this will give you 2 6disk raidz2 pools. this avoids having to deal with any extra cards or anything else. this also has the benefit that you would not have to move your data off the box.
also some others might come on here and tell you to avoid the marvell points on that board you have like the plague. I have a very similar board (c2550D4I) and have had no problems with my marvell ports. It is true they are not as reliable as the intel ports and ASRock themselves say in the manual to use the intels first and then use the marvell for the reliability reason. But before you spend money on extra cards, I would try to use the marvell ports for a couple of weeks or months when you get your new disks added and then see how they behave. if no problems then you saved yourself some money. if not, then you can go the expansion card route.
my 2 cents!