You can make a calculated measure in PowerPivot to handle this. If you want to get the cumulative sales for all time you can do this:
CALCULATE( SUM( FactSales[SalesAmount] ),
FILTER(
ALL( DimDate) ,
DimDate[Datekey] <= MAX( DimDate[Datekey] )
)
)
If you want to be able to select certain time period (ex: running total for selected weeks or months) you can do this:
CALCULATE( SUM( FactSales[SalesAmount]) ,
FILTER(
ALLSELECTED( DimDate),
DimDate[Datekey] <= MAX( DimDate[Datekey] )
)
)
Source: Javier Guillen's blog