Visual Studio 2008 with Business Intelligence - Reports - Show a Parameter like a Tree.

Question

Hi, i'm working making a Report  in Visual Studio 2008 with Business Intelligence. I have this problem:
In my sistem i have the fields "Area Path" and "Revision". This fields are related as follow: 

Area Path 1
        Revision 1
	Revision 2

Area Path 2
        Revision 3
        Revision 4
        Revision 5

I want to show this information using a Parameter in combo box like a tree, in wich the user can select multiple values (values of "Revisions").

But Visual Studio offer the Parameter types: Text, Boolean, Date/Time, Integer and Float. Someone knows how can i implement this Tree in one Parameter?

Thanks!!! 

Pablo Magariños Lucca