noahark2, may I suggest that you post this situation into the general forums at oldhouseweb.com. Lots of the homeowners there have done foundation/support work and may have some experience in your area.

Hi,
You strip off the siding, and bolt jacking plates to the post at the second floor line. The jacking posts go under these plates (think: upside-down ledger) at a slight angle. you want to have these posts heavy enough so they don't deflect or bow, but a-frames work, too. The jacking posts or aframes go in on both exterior sides of the post, so the thrust is applied in both directions, and the angle of the jacking bearing points on the ground should be far enough away to permit working space. If it would work better in your situation, you can attach them up at the second floor plate, but as the jacking posts get longer, they must get heavier to resist bowing, and over 16' you're looking at a rather unweildy timber to do the job.
For extending a corner post, you need some kind of shouldered slip joint, or steel plates and bolts.
Have fun and be cautious.
Casey

I'm not a contractor nor an engineer so I don't really know what I'm talking about, LOL. But what I would do is contact a supplier of recycled barn post and beam. There is a good one in PA I could direct you to. I asked a lot of questions and found out some info.

From what I understand most post and beams are tightly fitted with mortise and tenon and pegs. With the rot at the post you may have to cut part of the post along with the sill and then get someone who knows what they are doing to re-fit or explain exactly how to fit the post and sill back together.

This Old House had an episode where they jacked up a post and beam house, cut out the rot in the sill and lowered it back down. To jack up the house they had to build a false internal wall attached to the structure at the top (3rd floor?) and place the jacks in the basement below the false wall. If I remember correctly the reason they had to do that was because the floor joists where just sitting on the sills and a beam spanning the center. So jacking up the floor would just raise the floor rather than the structure. I'll see if I can find that episode. I remember thinking I wouldn't want to try that DIY.