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Currently there is no support for adding external nodes to our DOKS offering. All of our provisioning and configuration is heavily dependent on our bootstrapping process. That process would not occur or be guaranteed when allowing external nodes. This would also bring up issues with supportability and troubleshooting clusters as DO would have no way of reconciling or upgrading those nodes.
Sorry for any inconveniences this causes you.
Regards,
John Kwiatkoski
Senior Developer Support Engineer - Kubernets
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