You just keep doing add(key, value) calls:
ModelNode address = op.get("address");
address.add("subsystem", "messaging");
address.add("jms-topic", "myexampletopic");
You can discover the legal types (e.g. "jms-topic") for children of the subsystem=messaging resource via the read-resource-description operation described above (the legal keys are the names of the child elements under "children" as described in "Description of Parent/Child Relationships" above). You can also execute the "read-children-types" operation against the subsystem=messaging resource; the response is a list of valid child types.