I have install Rancher using the single Node Docker. It it possible to import and manage DigitalOcean managed Kubernetes Cluster? I don't see any option in import except for EKS, AKS, GKE and Generic.

Yes, if you select Generic, you can import a cluster from Digital Ocean.

I tried but it just hang on provisioning. By the way I am using the DigitalOcean DOKS (Kubernetes Managed Service) option.

It hangs during provisioning if the cluster agent on the imported cluster cannot connect to Rancher. This most commonly happens if the Rancher server URL was set to localhost, but can also be caused by security settings on the cloud provider side. The port requirements are here https://rancher.com/docs/rancher/v2.6/en/installation/requirements/ports/

So if my rancher is sitting inside Corp firewall then I have to open up 443 for access.

Yes. If it is inside a firewall you can install Rancher behind a proxy https://rancher.com/docs/rancher/v2.6/en/installation/other-installation-methods/behind-proxy/

So if it is something like digital ocean kubernetes service where I have no access to the control plane , does importing to rancher still works?

Yes, it should still work.

Thanks. What is the difference between importing and having Cluster Driver like AKS?

The main difference is that when you delete an imported cluster in Rancher, the cluster is not destroyed. But if you create a cluster through Rancher and delete it in Rancher, the cluster is destroyed. Imported clusters used to have fewer features but they have feature parity now, so they are a really good option.