The package ships with a BackupManifestWasCreated event that enables you to add additional files to the backup zip file.
When backup process starts, the package will create a manifest of all file that are selected for backup. Once the manifest has been created, a zip file is made containing all the files in the manifest. The zip file will be copied to the backup destinations you configured.
However, if you have cases where you need to add additional files to a particular backup, you can do so, between the creation of the manifest and the creation of the zip file.
Right after the manifest is created and before the zip file is created the Spatie\Backup\Events\BackupManifestWasCreated event is fired. This is what is looks like: