You can indeed run “qq input.xml| jq '.' | qq -o hcl”. The tool does aim to serve that purpose that’s why it outputs json by default. The codec is importable as well but there’s a lot of work to be done improving and maintaining it.
$ qq tests/test.hcl | jq | qq -o yaml
app_name: SimpleApp
database:
- host: localhost
password: password
port: 5432
username: admin
...