Yes, it is possible to include helper blocks in your extension. @BeksOmega has previously provided documentation on how to do that here (see §1.1). I have used these in a small extension that connects to some BLE-enabled scales in order to represent the supported units here.
As for the extension template repository, you need to clone it and provide your Java classes defining your extension following the guidelines in the extensions document. The necessary dependencies are listed in the README and there is an update-appinventor.sh script you need to run the first time to complete the rest of the setup process.