Instead of using a Copy Column transformation, use an ActiveXScript transformation. Click on the properties button of this new transformation to edit the code. Enter something similar to the following:
If IsNull(DTSSource("Col001")) Then
DTSDestination("Name of Destination column") = " "
Else
DTSDestination("Name of Destination column") = DTSSource("Col001")
End If
Main = DTSTransformStat_OK