I should clarify on your last question. My posts have been encouraging you to add 10 more disks, create a new disk group, and add it to the pool. This effectively doubles your usable space and performance potential. However, your last question was "can you add two more disks to the vdisk already in pool A". The answer is, yes, you can add invdividual disks to an existing vdisk up to the limits of that RAID type. In the case of RAID 5, you can have up to 16 disks in the disk group. So, you can technically expand your disk group rather than adding another disk group to the pool. I'd caution you though to think about why you'd really want to do this. The whole idea of virtualized storage is to make administration easier and get aware from the mechanics of micro managing RAID sets. If you knew for sure that 2 x 600GB drives were all you needed to get you through the next couple of years then, ok, maybe expand the vdisk. However, you'll have greater chance of staying out of trouble if you come up with some reasonable growth size that you can buy disks in and use the pool mechanism instead of trying to manipulate existing RAID set sizes.
One good reason to edit the size of an existing RAID set is to try and get that RAID set (vdisk) to a "power of 2" as detailed in our best practices guide. If you haven't read that document, take a look at https://h50146.www5.hpe.com/products/storage/whitepaper/pdfs/4AA4-6892ENW.pdf