Anyone on API Connect 2018 I recommend you move to v10. But if you are unable to and need to apply some configuration to DataPower you can apply configs like below.

Thanks to John Bellessa for his help

1. Create you datapower config and put it into a text file. In this example we will call this datapower.cfg
2. Create a configmap kubectl create configmap custom-dp-config --from-file=datapower.cfg

3. Create an extra values file. In this example we will call this dp-extravaluesfile.yaml

   yaml datapower: additionalConfig: - domain: apiconnect config: datapower.cfg

4. run apicup subsys set <gwy> extra-values-file dp-extravaluesfile.yaml
5. run apicup subsys install gwy

THIS IS ONLY FOR APICONNECT V2018 not V10