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OK, I think I get it. My single node can already be used to schedule user workloads. But the cluster was not created by Rancher, and therefore Rancher cannot extend it. If I want to add additional nodes, I need to do that manually, either by adding additional server nodes or by adding agent nodes.

^ correct, new nodes need to be added however you created the first node. It’s also not recommended to run user workloads on the same cluster as Rancher.