Hi Vinay,
Well there's a lot of "it depends" in that - especially depending what is already in your landscape. Your best approach is to contact your SAP Total Quality Manager if you have on, or go to your local SAP sales team if you don't, explain your scenario and get them to check for you.
And yes it is possible to add/remove/move columns and attributes in both the BPM and Gateway Unified inboxes - on the organizing and personalizing your work items front I would rate it as a somewhat better than the UWL... there's a bit more power in using the UI5 than the old UWL XML.
Hope that helps
Jocelyn