Pretty sure you would need a Boeing (or equivalent), short type rating course with an FAA oral and check ride. The checkride would have to be done in an FAA approved simulator as an example, Boeing Gatwick, Singapore, Miami, and perhaps Shnghai or Inchon. Don't know if Boeing is doing any "walk in" non airline training for the 787 at the moment. They don't make this easy! Might also check with AA or UAL. If they have a Part 142 school they may have someone who can help. I assume you know that in the US, the 777 and 787 are separate type ratings.
Good luck