Dura Ace 9000 Mechanical to Di2 ?

Yes you can, is the short answer to your first question. Seat post can take battery without issue.

Not sure what you are asking in your second question? Instructions on how to fit Di2?
Not sure what current options from Shimano are for Di2 handlebar ports.

I built up an F8 for somebody a few years back. Fitted wires before anything else. Needed a special seat post internal battery mount for Pinarello aero post which had a sleeve some washers and a lock clip. I put junction box under bars (charging port is on junction box). Also needed pinarello electronic fitting kit which was plates and grommets for cable exit points on frame.

(IMHO if it is a pure dry weather bike, and unless you have loads of spare money, I wouldn't bother with DI2...just my opinion though)

Somebody else may have more up to date advice though