Hey Taro,
sorry, but I never had this problem before. But I found a similar requirement as yours in the forum maybe it will give you an idea how to solve your issue. Here is the link:
short term portion of a long term loan to be po... | SCN
Prasad Alora Veedu posted an answer on the 25th of May 2010 11:53 AM concerning a possible workaround for money market but I don't know if this is a real valid approach.
From my point of view a workaround would be to do a repayment of the partial amount that you would like to transfer to a new account assignment reference. Then create a new position (with the right AAR) which has the same cashflow as the repayed position. It's just an idea. I never tested it.
Regards
Robert