I'm building an attached deck and have already buried precast piers in the ground. Unfortunately, I had to dig so deep that the tops of the piers are below finished ground level. I'd rather not bury the 6x6 posts. The tops of the piers are 8"x8". Should I use sonotubes and pour concrete on top of the piers (assuming I can jury rig round sonotubes on top of a square pier top) to bring the piers above ground level? How about putting a post sleeve around the post to keep it away from the soil? I know metal and today's PT lumber aren't friendly with each other, so could I use plastic sleeves?
Without seeing the plans and what kind of precast piers you have - I would say that I'd pour more concrete, with some rebar drilled and epoxied into the precast piers and the new concrete poured over to join them both.
Then set stand-off post bases into the new concrete for the PT posts.
Pour your concrete first and set the preformed pier on of the concrete to get the post bottom about 4 inches above grade. No sonotube needed.