I got it working, as described in section "add new orderer node" of my project: https://github.com/yxuco/fabric-operation/blob/master/operations.md
You need to update the system channel config with new consenter info (which includes base64 encoding of the new orderer's certificate).  I used the following jq filter to edit the channel config:
cat ${chan}-config.json | jq '.channel_group.values.OrdererAddresses.value.addresses += '"${addrs}"'' | jq '.channel_group.groups.Orderer.values.ConsensusType.value.metadata.consenters += '"${cons}"'' > ${chan}-config-modified.json

The limitation is that you can add only one node at a time, even though in principle, you should be able to add multiple nodes as long as you have more nodes running.  But, at least for the release 1.4.3, it allows me to add only one node in each update.

Besides, you have to use an orderer user to update the channels.  You have to update the system channel first, and then update other application channels separately.

We have the same question, anyone can help us??

We are using fabric server to generate certs and use raft.  Is there any reference to add new orderers?