Yes, if you wanted to convert the amounts to YTD balances in BW, you need to implement a logic in end routine to convert the amounts to YTD. Below is the logic,
loop through the result package,
for every record, take the amount and post in all the next fiscal periods/months,
append the new records to result package
end loop.
how it works, with an example,
100010 001.2015 2000$ posted in Jan, while it is getting processed, you will create 11 extra records in to result package, that means, you will post 2000$ to all the next 11 months.
then in the next month, you get a record for the same account, 100010 002.2015 3000$ the logic will do the same, this time it posts 3000$ to all 10 months. So basically it is a cumulative logic, if you select any month for reporting, you will get the YTD balance stored in the month.
Note: Make sure that, Balance keyfigure is created as exception aggregation last value, with reference to char, fiscper.