what sort of extensions do you have in mind?
In general you could add your custom REST endpoints (using JAX-RS and CDI) and simply drop them into the jbpm console war file and they should be loaded by the server.
I would extend the REST API used to manage the processes and tasks. For example, I would get the form of an user task by a REST call
you can use following REST API, it generates an URL to show the task form on a remote application.