Tools, New Config.
Adding a SATA HBA would be a viable option, but if you aren't going to need to add much capacity, it would be cheaper and faster to just use larger drives. My suggestion would be to add an 8TB as parity, then when you run out of space upgrade one of the remaining 500GB with another 8TB, and you would be set for a while.
BTW, parity is NOT backup, it's high availability. It allows you to continue to use your server uninterrupted if a drive fails and you replace it. Parity can NOT repair corrupt or deleted files, or fix a corrupted filesystem. It only rebuilds a failed drive with exactly the content that was on it when it failed, if the content was corrupted, the rebuild will be corrupted as well. Parity also relies on ALL the remaining data drives to be read perfectly from end to end, so you must be sure all your data drives are in perfect health. Since some of your drives are over 10 years old, that may be questionable.