I now like to add two more hard drives and get more space, and I would like to know after I have added two new drives, will I be able to migrate to RAID 6? and Can I use different brand of hard drives?
A brief search shows the Thecus N7700 is capable of online RAID level migration i.e. not lose data while still accessible during the migration.
Whether it's capable of online RAID5 to RAID6 migration you'll have to read the manual and see what it says.
RAID migration is supported, but only from RAID 0 to 1, 0 to 5 and 1 to 5. You can add a hot spare, however, to RAID 1, 5, 6, or 10 volumes.