Thanks for the reply.
I saw this article. It only tells me how to add a VM to the existing cluster. With this approach, I will need to create a new VM and add the VM to this cluster with DEPLOYMENT-ID and RESOURCE-GROUP-ID. I will also need to put the new VM to the same subnet as the existing mongodb VMs.
This is the most straight-forward way. I'm just wondering if I could change some current config, for example, change slave machine from 1 to 2 so Azure will automatically create a new VM?