It is not possible to use variables as drill object in Webi reports. Here is a clever trick how to overcome this limitation using alerters:

http://deltekbpm.blogspot.dk/2013/03/be-alert-part-1-to-drill-or-not-to-drill.html

The idea is to define a variable that calculates the drill level using the function DrillFilter() and then to use this variable in alerters to “overwrite” the value of the drill object with the value of the desired variable.