Hi @S__k__Agarwal
To hide and remove the unwanted columns, you can use the approach suggested by @Himanshu_Jain .
To add a custom column "Country", you would need to overlay the following path /libs/wcm/core/content/common/availablecolumns and add the node for the Country. Along with that you would also need to overlay the following script to write the custom logic to populate the country value.
/libs/cq/gui/components/siteadmin/admin/listview/columns/datasources/availablecolumnsdatasource/availablecolumnsdatasource.jsp
Hope this helps.