Expand RAID 5 Smart Array P410i

Greetings,

We have a Proliant DL 360 G6 running with Win2K8 R2 x64. OS running on RAID 1 (2 disks) and DATA on RAID 5 (4 disks), both without Write Cache.

Now I want to expand the RAID 5 with 2 more disks, but I don't see any options in the ACU for disk expanding. I do see an option to assign them as spare.

Please advise.

Thanks.