@jakobdoering for current Beats and modules the component templates won't help you: I think only the data sources in Elastic Agent are using component templates.
But I did find the documentation for adding fields to Beats templates, which you can use right now. The config option is setup.template.append_fields and is documented at
https://www.elastic.co/guide/en/beats/filebeat/current/configuration-template.html
Note that when adding fields this way, you'll have to run your Beats setup again, so the index template is updated.