During a data load, by default, Essbase overwrites database values with data-source values. However, you can add data-source values to database values. For example, if you load weekly data values, you can add them to create cumulative data values in the database. The data source is not changed.
If you are not using committed transactions, adding data-source values to database values complicates recovery from database interruptions that occur while data is being loaded.
To add to existing data values during a data load:“Adding to and Subtracting From Existing Values” in the Oracle Essbase Database Administrator's Guide
import data (aggregate storage) (MaxL) in the Oracle Essbase Technical Reference