The prisoner is allowed to ask for up to 3 defense witnesses. Generally speaking, however, if he asks for other prisoners as witnesses, they are disallowed. It's all window dressing anyhow. In other agencies these employees are usually educated, tested, reasonable and fairly knowledgeable. In those agencies they are called administrative law judges.
Lets use a purely fictitious example.
Lets pretend that, like most of the HEs, skunky Chunky was a prison guard before slithering into this much better paying job. Like almost all hearing examiners, Chunky was a guard and hates prisoners. His duty will be to impartially hear the charges made by a guard against the prisoner and to fairly render a decision. He's handcuffed, belted or chained and sometimes shackled. HEs are terrified that after they screw some prisoner, the man might be a tad miffed and elect to remodel the hearing examiner's smug mug.