I am afraid jBPM does not have such functionality at the moment, I mean just to inform it about JNDI name for transaction manager. It has several fallback names configured but they are final. With slight modification to the code base it could be done, but probably you are interested in changing the code base, at least not at the moment, right?
Since you know the exact JNDI name you can just look it up and set on the environment when creating sessions.
P.S.
Could be a good candidate for an enhancement to provide a mechanism that will allow to set custom JNDI names for some runtime components, such as transaction manger.