I have a 16' x 8' 2nd story deck with 10' posts. It is attached to the house on one 16' side and has stairs off one of the 8' sides. The posts are placed at the edges of the concrete on the pad below. The thickness of the concrete is approx. 4". How do I go about adding a proper pier/concrete pour to make a legitimate support for these posts? I really want to avoid removing the posts if at all possible and would be great if it looked good in the end as people are constantly in the backyard. Thanks for the help.

Looks like the deck may have been build without a permit because you do need a pier as you have noted. I can't see how the posts are connected to the deck above so I can't tell if they can be moved forward into the yard by about 6". I would support the deck with a temporay post on each side of the current post, remove the current post and break out a 1'x1' square or cut with a cement cutting bladed saw. Dig down to your frost line and make the hole slightly larger at the bottom than the top. Fill the hole with concrete and trowel flush with the existing slab. Set your hardware into the concrete while wet so you can attach the post bracket in 2 days.

To add onto the deck to move the post out would be a major undertaking.

You need to use felt expansion joints against the 3 sides of the floating slab so it doesn't bind and interact with the new "stationary frost footer.