I once tried the little Hang Seng contract. Had to use Idealpro on IB to exchange USD for Hong Kong dollars.
Then i could trade the futures contract which settled in Hong Kong dollars. No USD settlement.
if you have a choice maybe treat everything as a spread trade. Buying Nikkei with Yen is selling yen
and has been a good opening trade lately. Closing is selling Nikkei and buying yen.(covering lower)
Opening a short Nekkei positon with an account funded with USD is effectively USD/Nekkei and
that recently has been the way to go as well. Of course i am focusing on the USD leg being sold
to close at a higher price. Maybe the short Nekkei leg lost more. I dont know.
I am talking of a 401k account where you cannot borrow. Dont know if this applies to you.