The video surveillance edit happens while the transport is still at the prison, so we know that he got out of the transport before it even left.
At this point we don't know exactly what occurs, but we do know that the warden and his men have been compromised due to Carroll's people having the warden's daughter, Dana in captive. The lawyer is is also being blackmailed into helping Carroll and she can easily get through prison security with her car without a lot of questions.
Based on all of this, we are meant to assume that Carroll was secretly smuggled into the lawyer's trunk. The specifics are simply not known but it given the situation, not entirely impossible.