kluctl uses kustomize to render final resources. This means, that the finest/lowest level in kluctl is represented with kustomize deployments. These kustomize deployments can then perform further customization, e.g. patching and more. You can also use kustomize to easily generate ConfigMaps or secrets from files.
Generally, everything is possible via
kustomization.yaml, is thus possible in kluctl.
Using the Kustomize Integration
Please refer to the Kustomize Deployment Item documentation for details.