Hello,
May be you try this:
Create a info object(ZFACTOR) with Numbers only as Data Type and use this ZFACTOR in your cube and also in aggregation. Create a variable ZVARIABLE on info object(ZFACTOR) and use this variable ZVARIABLE in your filter.
Use ZVARIABLE where user can enter his exchange rate value and use this exchange rate in your fox formula to convert to JPY.
Thanks
Anand Kumar