Derivative Rules are the normal way of aggregating source data records. The usual way of using them though is to map the net position ie you aggregate the source into a single record which then gives you a negative or positive amount. You then map that as a credit or debit to the relevant account(s) and set the source records to Bypass. I am assuming that you want to map the source records and not the aggregated derivative record. I would create a derivative record to execute the aggregation. Set the Derivative Type to Interim so that it does not get loaded to the cube. Then on the Account dimension create a map (not One to One) with the Logical Operator set to Business Rule (I don't think a Complex Expression will work on Derivatives but I might be wrong). The Business Rule reads the value of the Derivative record; if it's zero the result is Bypass, if it's not then the result is the Account Cube member.
