How to create a 'System'variable to identify the system from merged dimension

Hi,

     I have a webi report with merged dimensions on users and
roles. Users and Roles are coming from 3 different systems (ECC,BW,BOBJ). Now,
I don’t have the system field available in the queries to filter by the system.
So, I would like to create a variable for system where I want to be able to
identify the system (like ECC,BW,BOBJ) based on the merged “Role” dimension.
Could someone tell me , if it is possible to do that? And if so, how do I do
that? (I tried, but it's not working for me) I really appreciate your help.

Thanks,

Charvi.