How can I convert message from MQTTTransport?

Hello,

I'm using ActiveMQ with JMS and MQTT, like following.
clientB - JMS - AMQ -(*)- MQTT - clientA.
I want to convert data in message only from/to MQTT at (*).
The conversion depends on authentication information.
I tried to convert data by modifying MQTT plugin.
But I can not find how to get SecurityContext on MQTT plugin.
My question is
How can I get SecurityContext in MQTT plugin?

Regards,
mtakahashi