In the cube settings you can use a translation algorithm type of "Standard Using Business Rules for FX Rates".
Once this is set, you can write a Finance Business Rule and add it to the cube settings. I don't have any non-client code samples available to post publicly, but examples of what I have used (or seen used) are to set FX rate for a currency, such as VEB, to 1, or to set FX rate for a specific scenario to 1. Another example is to combine FX rates for a forecast scenario, such as copy actual for Jan to Mar, then copy Budget for April through December.
You can also set rates in a specific scenario for a specific account type (income/expense) to 1 while leaving the other account type (asset/liability) alone.
If you need to get more granular, then the setting on the cube is Custom for translation algorithm. Here you can set rates or translation methods for specific accounts, such as cashflow type accounts use a YTD average for changes in account balances instead of monthly averages.
Hope this explanation helps.
