There is no built-in support for reading or sending JMS-messages. However, BPMN has a notion of message-events and signal-events. Using a correlationId that is stored in the process-instance business-key (can be provided when started) you can easily lookup the appropriate activiti-process using our API when a JMS-message has been received and signal the process to carry on (optionally passing in payload based on the message). For sending JMS-messages, you can create a JavaDelegate that queues a message on JMS and model that in a service-task in your process.
Off course, using the camel integration for this makes a lot of sense, as Camel has a lot of stuff built-in for handling queues and JMS.