currency conversion for average period like from 5 to 10

Hello, we have a currency conversion issue like that:

If we take report with period interval from 1 to 10; the currency should be calculated by average currency of ten months, and if we take it from 5 to 10, the average should be calculated for five months. And if user takes the report for only one month, then it uses average of the month.

I mean, if user gives a period it uses period average, and if he/she gives a month it uses month average.

It is possible to do it in BW with Rscur?