RAID5 is hardware level on the motherbaord or your RAID card and is not affected by your OS being used. If it says RAID5, then RAID5 is what you have been using.
RAIDz is the ZFS equivalent software-RAID to RAID5 where RAIDz2 is more similar to a RADI6 setup. Simply load up FreeNAS, select 'Import' under drive management, and select the RAIDz pool that you want. It should pick up all of the drives (and your data) automatically through the import process.