Hi Vinay
As far as I am aware custom attributes for BPM is only possible with the UI5 inbox options - either BPM Inbox or Gateway Unified Inbox. The same feature was not provided for UWL and I have not heard of any plans to do so.
If you need custom attributes and columns for BPM then best current advice is to move to one of the UI5 inboxes. The Gateway Unified Inbox would be the closest to the UWL as it supports both workflow and BPM - but be aware that this is not an exact feature for feature match so you need to do your due diligence if you want to move across. In other words the Unified Inbox provides more features in some areas but does not support other scenarios that are part of UWL so you need to assess what features you actually require.
Rgds,
Jocelyn