Using SQL Server Integration Services to Load Data - Microsoft SQL Server 2008 Analysis Services - page 412

After you map the source column to the destination column, you no longer see the
source column in the drop-down list. If you have a composite key in your dimension and
you need to map a single relational column to several destination columns, you must
tweak your OLE DB Source component to have a single column appear more than once
in your OLE DB Source component. In the OLE DB Source editor, define the data
access mode as SQL Command rather than Table or View (as shown in Figure 22.4).
Then, compose a SQL command so that a Select statement contains the source
column more than once. Alternatively, you can use the Derived Column transform to
duplicate columns.