I fear there is no setup guides applicable for your szenario.
However there are some documents, which gives you a very good impression on what to do:
For how to extend nodes (especially on primary):
But this also accounts for standby (except the instance management part, this is not applicable). To get an idea on what to do (srvctl commands) see here:
the procedure of adding a node should be same on primary and secondary clusters except for the part that add the instance/thread to the existing cluster database. I would have just created a new standby controlfile from the primary after adding the instance/thread and copy it over to the standby.
its a pretty straight forward process.
1) Prepare your new nodes ( As you prepare your RAC nodes for the new installation, include running cluvfy etc.)
2) Run add_node.sh for clusterware
3) Run add_node.sh for Rdbms
4) Add the new instance/thread to the cluster database. ( be sure to add the new database instance to CRS if you are not using DBCA to add the instance)
you may run into errors during the execution of add_node.sh, but there is much info in google about those errors :)
Also, be sure that you remove passwords for your ssh key files if you have set any, ssh-agents doesnt work for node adds.