Assuming the new host that will be added to the existing cluster is the same server, I have a few questions:
1. Do I have to disable first HA/DRS before I add the new host?
2. If the cluster is using distributed switch, how do I configure the new host to use the existing distributed switch configuration?
No need to disable HA or DRS.
After you add the host to the vCenter(and cluster if needed), you can following this KB to add to the existing vDS:
vSphere 6.0 Documentation Center
Hope this helps