I plan to use the above docs to add a new 12GB/s SAS HBA, but wasn't sure if there were any gotchas since I can't seem to find any docs for adding a new HBA to an existing FAS 8020 that didn't already have one.
The only thing that I would recommend is reviewing the NetApp Hardware Universe to verify the slot priority for the new HBA card. Based on the platform it would probably be slot 1 and if slot 1 is in use then slot 2.
After you install the card, you may want to boot into maintenance mode (boot menu, option 5) to verify you can see the card.
Let us know if you have any other questions or concerns.