Ron,
Currently there isn't an automated way in RAFW to accomplish this task. We do have a request for enhancement opened internally for this enhancement and I will add a note indicating your interest.
For now you will have to perform this operation manually. If you nodes follow a pattern you can copy existing node directories in the rafw_home/user/environments/envname/cells/cellname/nodes directory.
For example, copy the node1 directory to nodeNN directory. You will then need to update configuration files in the tree. In the cell level configure.properties file you will need to add the new node to the list of nodes in the NODES property.
In the newly created nodeNN directory you will want to look at the install.properties and configure.properties files, if your nodes are created following a pattern, the updates should be minimal. You will also want to updated the servers directory(ies) nested in the nodeNN directory.
For new clusters, you will want to follow a similar pattern for the rafw_home/user/environments/envname/cells/cellname/clusters/clustername
Updating the cluster.properties file to specify the correct nodes.

Additionally, you will need to update the Environment Group in Build Forge to reflect the changes made to your RAFW environment.

Hi Tim, I will also be interested to know when this enhancement will be introduced into BF RAFW. We have similar needs to scale horizontally.

Thanks,

Hi Tim, we also have a large insurance customer in Columbus that is interested in this enhancement request.

Also, after following Tim's steps of configuring the xml and properties files, you'll want to know what commands to execute. To add a new federated node you'd:
rafw -env <env> -cell <cell> -node <new_node> -t was_70_install_managed_profile

rafw -env <env> -cell <cell> -cluster <cluster> -t was_common_configure_create_cluster (this will re-create an existing cluster too)