Per Koel documentation, there are a few ways to do this manually via CLI and/or cron entries.
However Koel also provides a couple of options to do this automatically:

1. (Best) Watch the Media Directory
   This entails to include an additional linux package and the setup and the background run of a script
2. Command Scheduling
   Install (add to the cloudron package) and allow the Koel built-in scheduler to run.

Alternatively, it is still possible to add Cron commands to the Cloudron package, "manually" running specific Koel CLI commands. However, as mentioned in Koel's doc:

Though you can still manually set up cron jobs for individual commands, the scheduler is the recommended approach to do command scheduling in Koel, as it will automatically cover any commands that may be added in the future.

Maybe this could be considered to improve the Cloudron application package?