Starting from WPBakery Page Builder (formerly Visual Composer) version 4.4 it is allowed to modify predefined grid elements. Select grid element you want to modify under Grid param window tab “Item design”. Choose “Modify selected” under drop-down list “Grid element template” which triggers opening “Grid Builder”.
Important: Modifying predefined grid element does not overwrite predefined grid element, instead making a copy of it. Do not forget to add title for your grid element. Your newly created grid element will appear in “Grid element template” list located in “Item design” tab of grid param window.