No Title of Nobility shall be granted by the United States: And no Person holding any Office of Profit or Trust under them, shall, without the Consent of the Congress, accept of any present, Emolument, Office, or Title, of any kind whatever, from any King, Prince, or foreign State.

He cannot "accept ... [an] Office ... from any ... foreign State." I suppose he might argue that "accepting" and "seizing by force" are not the same thing.

Or Congress could pass an act to allow it.

Edit: Or, more realistically, someone will complain to the Supreme Court and they'll find a reason to avoid ruling on the issue and Sotomayor will write an angry dissent but nothing will change.

It is supposed to be covered in the emoluments clause https://en.wikipedia.org/wiki/Foreign_Emoluments_Clause

It actually could be OK but congress will have to vote on it to allow it.