Not sure if it's that simple to have a secondary data_engine pointing to a database containing information of another domain than the data_engine is in itself. Then again, it might be that's not a problem and you just need to configure DB authentication and address on it. I'd be very careful with that though, just in case.
I'm not sure how having that data_engine would alleviate any reporting needs. All IDs will reset for all objects in your database (qos, robot, device, ci, etc) and they wont match to what you have in your old database. Moreover, data_engine itself doesn't really handly reporting or provide access for components that do: usually it just gives the connection string for other components.
What kind of reports are you looking to have from your old DB?
-jon