Hello,
there are no limitations for RAID 6, so you shuld be able to expand the RAID. First of all, make sure that you are running latest Controller Firmware and lastest Smart Storage Administrator Version.
The Smart Storage Administrator does no longer need to first expand and than extend like the old ACU. As long as I know, there is currently one action olny.
Open the SSA, select configure--> select the controller--> select the array you wish to extend and than click on "manage data drives" on the right side in the "actions" menu. The possible extend option will show up.