Hi, You can go with virtual info provider with Function Module
1. Create RFC enabled Function module on ECC system based on custom report.
2. Create Virtual provider on BW system with function module type
3. Inside virtual provider's function module , call that RFC function module from ECC system.
4. Now combine this virtual info provider with basic info cubes into single multi provider and in single query to compare data [like comparing plan vs actual]
You can refer SCN for more information on each steps.sure you get more details..
Let us know if you need further assistance.