You can add an existing Azure KubeOne cluster and then manage it using KKP.
Navigate to KubeOne Clusters page.
Click Import KubeOne Cluster button.

Azure provider.

Select a pre-created preset which stores the provider specific credentials.
Manually enter the credentials Client ID, Client Secret, Subscription ID, Tenant ID used to create the KubeOne cluster you are importing.

Import KubeOne Cluster.