How can multiple messaging systems be connected so that messages available on one are also available on the others?
Use a Messaging Bridge, a connection between messaging systems, to replicate messages between systems.
Another alternative is to bridge systems using Change Data Capture.
A basic bridge between two messaging systems (such as WebsphereMQ and JMS broker) can be done with a single Camel route:
<route> <from uri="mq:queue:foo"/> <to uri="jms:queue:foo"/> </route>