To add custom features to the existing default extension, we need to download that extension from file cabinet, and after that, we need to create a new extension with the coding standards. and later, we need to add all the file code to the newly created extension, and then we need to add the script to NetSuite, and deploy the extension, and activate the extension.
Note: when we custom the default extension, then the next updates from that default extension will not be updated .