How can i export all WebCernter data into XML in one go, rather than approving assets one by one and exporting?
yup. you can do this by "Approve Multiple Assets" option available in your publish destination.
click on "Approve Multiple Assets", Select required asset types and select query as "Assets of selected asset type(s) in site: <yoursite>" and click on Approve.
This will approve all your assets in one go.
hope it helps.